The best racing game for the GC out now would have to be both Burnout 2 and NFS: Hot Pursuit 2... Burnout 2 has the best crashes and sense of speed... HP2 has awesome looking cars and is still very fun.

Racing games I'm looking forward to are NFS: Underground and Mario Kart: Double Dash... F-Zero GX looks really awesome too, but I've never really been a fan of F-Zero, although I might still consider the purchase.

I like the feature in Underground how you can customize practically every part of your car... that'll be cool. And the graphics are just top notch.

EDIT: I might also add that Hot Pursuit 2 for PS2 is a completely different game to Hot Pursuit 2 for the GC/XB/PC. The PS2 version was developed by a company called Black Box (the same one that is developing Underground for all platforms), the other made by a development team in EA itself. Apparently, the PS2 version is far superior, and does not have framerate probs as the other version has. Although, I still think HP2 is really fun for GC (and PC for me), even though it is not up to scratch to the PS2 version.