ATI hands down.
Must I keep repeating this? FOR NEW GAMES, ATI IS WINNING. I'm not a fanboy, this comes from a year of researching a new computer. nvidia just isn't offering FULL DX9. They say they do, but the image quality for ATI is the best.
Also, in Half-Life 2, VALVe stated that nvidia cards will get degraded quality because they don't have the full DX9 feature set like ATI cards do.
Nvidia fans can say what they like, but there's no denying that the ATI cards are already ready for the DX9 games.