An awesome free online multiplayer race game is trackmania: nations forever, there's a torrent for it somewhere.
Just get Steam and download from there. Don't need a torrent. It's a free game with free multiplayer.
Trackmania is a
FUN game. Better yet, if you have those red and blue 3D-glasses, you can play the game in 3D. It's surprisingly... well, surprising. (..but the 3D-glasses might make your head hurt after a while).
Test Drive Unlimited isn't too bad. Certainly a massive game. I thought the cars were a little hard to handle. I also thought the PS2 version might be better, but it's worse.
For more of an arcade game, I can't say anything about GRID (x850 lacks PS3.0), but the Need for Speed series is fun (though I didn't like Pro Street as much). FlatOut and FlatOut 2 were fun games too.
Has anyone tried Street Legal Racing: Redline? I think it's surprisingly good for an Activision Value game. I'd really like to see one of the bigger game companies take a spin at the concept. Handling isn't great (unless you have a wheel, then it's just hard).