At work I am told to turn off my desktop PC at the end of the day. My wife’s office tells her to leave her computer on overnight. My former employer said to turn off the monitor but leave the PC on. What’s right?

Given the energy, environmental and economic crises, my view is that you should completely shut down both your computer and your monitor any time they will be unused for more than a few hours. And, of course, you should set them to go into sleep mode after they are idle for far less time than that. To be honest, I myself am not always faithful to that advice, but I am resolved to get better at it.

Of course, there are some nuances to the issue of overnight shutdowns. If you, or your company, routinely perform important operations overnight, like backups or security scans or system tune-ups, then you have to leave the computer on, but you should still turn off the monitor.

How much memory do you recommend for a new computer running Vista Home Premium?

Two gigabytes minimum, and three gigabytes if your budget can handle it. The latter is roughly the maximum that popular consumer versions of Vista (called 32-bit versions) can use.

For smooth, quick operation in common tasks, it’s been my experience that Vista demands more memory than either Windows XP or Apple’s Leopard operating system. And the need for added memory rises if your PC uses “shared” video memory, which means your graphics system lacks its own dedicated memory and relies on draining the main memory. This arrangement is common in less expensive computers.

We are planning a trip to the British Isles this summer with a side trip to Brussels. We plan to bring our iPhone on the trip. What is the current situation with using European networks? Will my U.S. AT&T account just work or do I need to make pre-arrangements? I realize my iPhone won’t be able to access the faster 3G networks over there, but can I use slower networks? Being limited to Wi-Fi would be a bummer.

The iPhone 3G can indeed handle 3G networks in Europe, though the original iPhone can handle only slower networks. Both models can use Wi-Fi there.

You will need to call AT&T before you leave, though, and “provision,” or set up, your account, for international roaming. Note that such roaming is expensive, with very high rates for both voice use and data use. AT&T has some monthly add-on plans that cut these fees, but, if I were you, I would turn off data roaming until and unless I needed it; do most email and Web surfing via Wi-Fi; and consider downloading Truphone, a voice-calling program that uses Wi-Fi instead of cellphone networks.

You can find Mossberg’s Mailbox, and my other columns, online free of charge at the new All Things Digital Web site, http://walt.allthingsd.com.

At an event in London today, Ballmer said Microsoft (MSFT) will debut its own “cloud operating system” at its Professional Developer Conference at the end of this month. “We need a new operating system designed for the cloud and we will introduce one in about four weeks, we’ll even have a name to give you by then,” Ballmer said. “But let’s just call it for the purposes of today ‘Windows Cloud.’ Just like Windows Server looked a lot like Windows but with new properties, new characteristics and new features, so will Windows Cloud look a lot like Windows Server.”

Ballmer offered few details beyond that, saying only that it will enable “light editing” of Office documents. Presumably, that means Microsoft still doesn’t see products like Office moving entirely off desktop PCs and onto the Internet any time soon. I imagine we’ll be hearing quite a bit about Software plus Services later this month.

This week I took a look at some options for people who want to get rid of old electronics, one way or another. The good news is that there are a handful of Web sites that make it easy to do this — and some of them may even pay you for your old products. The bad news is that you’ll likely receive only a fraction of what you originally paid, especially if you waited a while to get rid of it.

Some sites, like Gazelle.com and VenJuvo.com, offer cash for your items and/or will recycle products. Another site, TechForward.com, lets people pay a fee to “lock in” a value for how much the site promises to pay for the product in the future. MyBoneYard.com accepts only laptops, desktop PCs, cellphones and flat-panel monitors, and gives Visa (V) gift cards rather than cash.

I was surprised to receive significantly different value offers from Gazelle and VenJuvo when trying to sell the exact same products on each site. In one instance, VenJuvo offered me $30 more than Gazelle for a digital camera; another time, I got $15 more from Gazelle for an old Apple (AAPL) iPod. It’s worth the extra step to shop around at more than one of these sites before getting rid of something.

Both ask a few questions about the item, including its condition and whether or not it still has the accessories that originally came with it. Gazelle determines a product’s value using retail — think Amazon (AMZN) and eBay (EBAY) — and wholesale channels; VenJuvo uses similar criteria and also looks at competitors’ prices.

If you worry about someone stealing your digital data, you’ll likely not feel comfortable dropping something in the mail that’s chock full of personal information, especially if it no longer powers on to allow the owner to wipe this information.

Both Gazelle and VenJuvo accept at least some types of digital cameras, laptops, MP3 players, GPS devices, camcorders and gaming consoles. Gazelle also accepts cellphones. But they don’t take everything. Gazelle doesn’t take LCD TVs and VenJuvo doesn’t accept satellite radios and portable hard drives or any smartphones or cellphones other than the iPhone; neither accepts desktop PCs.

I took the closest look at newly released Gazelle, owned by Second Rotation Inc., and walked through the simple start-to-finish process of selling a gadget and receiving money from the site. After pulling up the site, people can find their product and its value by choosing from a list of nine categories or by typing some part of the product’s name into a search box.

I sold Gazelle a first-generation iPod Mini with four gigabytes of memory for which my boss paid $249 in 2004. I answered a few questions about the product: Yes, it still powered on; no, I didn’t have the original AC adapter, manuals or software installation CD, and it was in “excellent” condition, according to my assessment. Gazelle placed its value at $25.

At this step, I opted to add the iPod to my box and check out, but users can also add other items to a box, including electronics for recycling. Gazelle’s policy is that it pays 100% of shipping costs for any box shipped to the company, so long as there’s at least one item in the box worth $1. Eighty percent of transactions qualify for a free box; the rest can be sent with printed-out prepaid shipping labels, but you must find packaging.

Gazelle lets users receive payments via a mailed, paper check or using PayPal; money is received either way within five business days. People can also donate their money to one of 23 causes, including the American Red Cross and World Vision. I opted for PayPal, and the $25 amount was deposited shortly after Gazelle received the iPod.

I sent the old iPod to Gazelle in a brightly colored, empty box that arrives at a customer’s door a few days after he or she sells the device to Gazelle. I secured the old iPod in the box using balled up paper, and sealed it with packing tape. A prepaid shipping label was already stuck to it, and I needed only drop it off at UPS.

If Gazelle receives a product and decides that it isn’t worth what you said it was — either more or less — and you’d rather not sell, the company will ship the product back, free of charge. But while Gazelle’s site guarantees users that they’ll receive their money, and that personal data are safe with the company, no money-back guarantee is offered.

Gazelle hopes to calm nerves by posting detailed instructions on the site about how to wipe a device of all private information. But the company hasn’t yet done this, and numerous users will remain skeptical even with such instructions.

I also poked around on VenJuvo Inc.’s Web site of the same name, www.VenJuvo.com, which is derived from two Greek words meaning “support, assist and delight sellers,” according to the company. This site, too, buys products back from people, though it pays via check, PayPal or Kmart (SHLD) gift card. Users fill out similarly simple questionnaires on each product to help assess value. Unlike Gazelle’s style of mailing boxes to users, VenJuvo gives users only prepaid shipping labels to print out and stick on a box that the customer must supply.

One notable difference between the sites is Gazelle’s broader range of products. In the case of digital cameras, for example, Gazelle accepts 80 brands while VenJuvo takes only Canon (CAJ), Sony (SNE), Olympus and Kodak (EK). Unlike with Gazelle, if you send VenJuvo a product that isn’t worth what you said it was, the company won’t return the product free-of-charge; instead, it will charge you for shipping.

If users choose to receive a gift card, they get a 10% added value. While VenJuvo doesn’t let people donate a product’s value to a cause, it will add this feature next week and will include different causes (like Ronald McDonald House and Big Brothers Big Sisters) than those found on Gazelle.

Unlike Gazelle, VenJuvo will always take items for recycling and will pay for the shipping, regardless of whether you traded something in for a value.

A useful resource for general electronics recycling is the Consumer Electronics Association Web site, www.MyGreenElectronics.org, which locates nearby electronics-recycling centers according to ZIP Code. And almost every computer manufacturer has a recycling program in place; some will even recycle computers that aren’t their own brand.

One way or another, it’s time to clean out the old junk drawer. Just be sure to do some comparison shopping if you want money for your old products.

Edited by Walter S. Mossberg

Write to Katherine Boehret at mossbergsolution@wsj.com