Netscape wunderkind Marc Andreessen, now about to turn a demographically middle-aged 38, has officially turned venture capitalist, a counter-culture move since the dot.com bust trashed the sport.

Andreessen and fellow Netscape veteran Ben Horowitz have raised a relatively modest $300 million – which seems to be considered a goodly sum in these dire times – and have set up Andreessen Horowitz as they’ve been publicly threatening to do since February.

Andreessen told the Financial Times they could have raised $500 million or $600 million but felt a twosome couldn’t handle that kind of money. They reportedly only intended to raise $250 million.

Andreessen and Horowitz have been playing angel investors for about five years now and have spread their personal wealth around some 40-odd start-up including Twitter, Digg, Ning, Rightscale and Linkin, some of them famous names that have yet to produce a return for their backers. Andreessen and Horowitz profess to take a long-term approach.

Andreessen Horowitz is proposing to be an all-purpose VC, willing to listen to any kind of deal that gets air time with it: founder shares, public stock, leveraged buyouts. They intend to make themselves available so they can keep their fingers on technology’s pulse.

The pair prefers Silicon Valley start-ups, believing that there’s a “special magic” in the geography that, say, the BRIC countries lack, and prefer firms concentrating on consumer Internet, cloud computing, software-as-a-service, mobile software and services, software-powered consumer electronics, infrastructure and applications software, networking, storage, database and other unidentified back-end systems.

It’s not interested in clean, green, energy, transportation, biotech, drug design, medical devices, nanotech, movie production companies, consumer retail, electric cars, rocket ships or space elevators, Andreessen says on his blog. Neither he nor Horowitz knows anything about that stuff.

Andreessen Horowitz says it stands ready to put anywhere from $50,000 to $50 million in the provocative start-up. It intends to back 60-80 seed-level fledglings (while valuations are low) as well as more venture-stage and late-stage companies. They’re looking for the 10 or 15 firms that will hit $100 million in revenue like Aliph, the Bluetooth headset maker they already have money in.

Andreessen Horowitz will only have two general partners but promises to build a professional staff. With as many investments as they have and intend to have, Andreessen and Horowitz themselves will be spread pretty thin. Raw start-ups can forget about them being on their board; as a matter of fact Andreessen Horowitz would advise them not to have a board at all. And they fancy founders as CEOs, hoping they grow into the role.

Horowitz hasn’t given up his day job as an HP VP and general manager of its business technology software optimization.

Andreessen has been telling the press that he expects half the VC establishment in Silicon Valley to crater and take their investments with them. “It’s not just that there are 400 extra venture funds. It’s that they are funding 4,000 extra companies, none of which will ever amount to something,” he told Reuters.

Netscape and Opsware, two firms connected with Andreessen and Horowitz, sold for a nominal $12.6 billion, more by accident that anything else.