nether get the 640MB GTS the 2900 is slow hot and uses to much power
new games are using >512MB for textures alone the 320MB card just cant keep up
and the 2900pro is slower then a 8800GTS to start with even with an OC your just catching it
a 8800GTS can do 600/1000 if you try and will much faster then the 2900Pro can ever hope to be
Actually the HD 2900XT is very competitive as of the latest drivers.
http://www.tgdaily.com/content/view/34344/135/
Before these drivers it would win half the benchmarks and lose the other half. This should help them win atleast 75% and i would not be supprised to see it take over the 8800gtx in a few test's as it was allready getting quite close (sub 10%) in a couple cases.