Depends on the resolutions, on resolutions like 1024x768 with medium quality settings on many games, the 7600GS, X1600 PRO will be enough, if you want more eye candy, you may go to the 7600GT, X850XT or X650XT and should be able to use the same resolutions with everything on high and Anti Aliasing, may be if you increase the resolutions you will have to slide down the anti aliasing a bit, we are talking about the most extreme cases like F.E.A.R., in other games, you should be able to crank up the anti aliasing with minimal impact. If you play on high resolutions like 1600x1200, or 1366x768 (Widescreen) or above, you should go for the 79X0GT(X) or X19X0 XT(X) etc. Todays games is not about capabilities, is about performance, look at the GeForce 6800 Ultra, with a SM 3.0 feature set and doesn't have the power to run next gen games with even the default SM 2.0 at a decent frame rate, and the "old X850XT" still able to even perform as the same or outperform in some cases the X1800XL, X1800GTO and the 7800GS and 7800GT in some cases. Look here
http://www.anandtech.com/video/showdoc.aspx?i=2746&p=5 and here
http://www.anandtech.com/video/showdoc.aspx?i=2746 and
http://www.anandtech.com/video/showdoc.aspx?i=2686&p=1 Those articles shows how close are each other, is just a matter of likes, If I were you, I would choose a card like the 7900GT, or X1900XT 256 or X1950 PRO, depends of your budget too, or the 7600GT, X850XT and the X1650XT.