Looks like ATI will be the way to go. The news yesterday and today is that even an ATI 9600 will kick major nVidia ass with the almost-released game. Apparently ATI has fully allowed DirectX9 code, where nVidia has not. Valve had to spend a lot of time creating a mixed-mode code path just to get some basic performance from nVidia cards.
http://www.anandtech.com/video/showdoc.html?i=1863
Lots of other sites with similar news online today.