While we're all waiting for GT4, might I suggest kicking off a round of NetKar on the ole' PC? Can't beat the price (free) and while there aren't MANY cars, they are from quite a wide range of genre's. (1 supercar, 2 streetcars, 2 open wheelers, 1 classic, 2 touring, etc) Superb physics... And network play works today. (I have a server I can set up again, it's currently down, but I'd be more than willing to race some people on here!)
The base program and bunches of newer (better) tracks can be downloaded from the
Official NetKar site.
