It’s finally official: Marc Andreessen has crossed over to what he once called “the dark side” and is now a venture capitalist.

Several weeks ago, BoomTown broke the news that the Silicon Valley legend and serial entrepreneur (pictured here) and his longtime investing partner, Ben Horowitz, had completed the raising of $300 million for a new venture fund.

And, indeed, the new firm–which is made up of just the two of them–is now launched and called Andreessen Horowitz. It has $50 million over the $250 million the pair had initially planned.

In an interview with me last week, Andreessen said that unlike many VC firms, Andreessen Horowitz will invest in companies at any stage of life–from early stage to late–and of any size and in any kind of digital sector.

That will mean about 60 to 80 seed investments, 15 that need following rounds and two to three late-stage companies.

In addition, the firm is able to take large equity stakes in public companies.

“We are unafraid of investing in 400 people instead of 40 people,” said Andreessen. “And we could invest $50,000 to $50,000,000.”

He noted the new firm would aim for areas the founders were more familiar with, such as the consumer Internet and mobile.

One certainty, though, was that the firm would focus on companies led by tech-savvy founders.

“We believe in entrepreneurs and those who live and breathe the tech product that they have created,” said Andreessen.

Andreessen said several major institutional investors–universities, for example–have invested large chunks in the fund, while a spate of tech luminaries has put in smaller amounts.

The quick completion of the fund raising, in the midst of a national econalypse, is a good sign. But the reputation of both longtime entrepreneurs seems to have helped.

Andreessen co-founded Netscape–the iconic browser company that was key to introducing the modern Internet to consumers in Web 1.0–and a lot of other start-ups.

He has also morphed into a mentor to many Web 2.0 entrepreneurs.

Andreessen made news when he announced on the “Charlie Rose” television show in February that he was creating the new fund.

“For the first time in my life, I am crossing over into the dark side,” said Andreessen at the time, in a joke about VCs being like Darth Vader.

Andreessen said he was essentially professionalizing the active angel investing that he and Horowitz had been doing.

Over the last several years, either together or apart, the pair have invested in a large variety of start-ups, such as Twitter, Aliph, Digg, LinkedIn and many more.

Andreessen is on the board of Facebook and an adviser to Twitter too.

Now, as a VC, he is upping the ante. “It’s important to have more capital,” he said. “Sometimes having a huge checkbook is a great thing.”

But, more important, he said, was to develop a knack for investing in successful companies with a fearlessness he said he learned from well-known VC John Doerr.

“John is always swinging for the fences,” said Andreessen. “I want to get into a cycle of backing radical ideas.”

Here’s a video interview I did with Andreessen about his new venture firm: