Some older games with lots of Porsches are Sports Car GT and Need for Speed Porsche Unleashed. Both are probably hard to find, but may turn up in the bargain bin at your local software store, or on Ebay.
SCGT is similar to the American LeMans series, with 4 different classes and a bunch or real tracks (Laguna Seca, Sebring, Lime Rock, Road Atlanta, Las Vegas, more). There are also other tracks and cars that can be downloaded, everything from 917's to Winnebago's.
Need For Speed Porsche Unleashed is only Porsches, You get to race Porsches from the history of the company, all sorts of 356's, 911s up to 73 (S & RS), 930's, 914, 944 (including S and turbo), and a wide variety of 964/993/996. Race cars available are the 550, 935/78 Moby d1ck, and GT1. This game has come up a few times before, search the archives for details.
Grand Prix Legends is pretty cool, but it has a much steeper learning curve.
"Pro Race Driver" is supposed to come out in a couple months, and looks promising. Hopefully you can skip the soap-opera crap that it appears to come with. No Porsches, unfortunately, but many cool tracks.
http://www.codemasters.com/redirect/redirect.php?url=/proracedriver/english/front.htm
For Playstation 2, Gran Turismo 3 is supposed to be awesome. Played it at the store a couple times, almost a good reason to buy a PS2.
Electronic Arts makes a F1 game, but I haven't played it. Someone needs to tell them that they have another game 95% complete and don't even know it. Porsche Supercup runs as support races for the F1 series. They already have the tracks and physics modeling done from the F1 game, and they have an exclusive licensing agreement with Porsche. They could put out the Supercup series in about a month, just by making the cars and the AI, either as an add-on to the F1 game, or standalone.
Tom