you have to know that the ATI HD X2900 did not came to compete with 8800gtx, it's competition is just 8800gts and she is a very good competitor. The price on both cards is very similar. If ihad to choose then my choice would be ATI. I think the card has lots of potential but not already explored on the current drivers. Thats jus my opinion.
BTW i'm a proud owner of a x1950xt and i love my baby so much.