I had an X850XT OC'd to 580 / 1230 with ATI Tool, and it was a fast card. The 850 also uses a 2 slot cooling solution which discharges heat through the back of the case. Since it doesn't create "heat polution" inside the case by recirculating hot air, the entire system cooling scheme benfits from the 850's cooling solution, especially critical when gaming and trying to keep an overclocked CPU running cool. Check out this THG article.
depends what you are going to play with it,
some games work better on ati, some preffer nvidia, most work the same on both.
but for an allround card, go for 7600!