Not sure about where you are buying, but in the USA, 7900GT stock is drying up and X850XT can be bought for
under $160 now. Newegg has 12-7900GT's, but 11 of them are out of stock. The one in stock is $334. I was just looking into eVGA's tradeup program as I have a 7800GT that still qualifies. problem is, all 5- 7900 series cards are out of stock from EVGA direct right now. (all three of the 7900GT's, and both the 7900GTX's) Also, only one 7900GT shows up on pricewatch right now. NV needs to get cracking as the first wave of cards is dissappearing.
Clearly the 7900GT is far superior to the X850XT. But they are in different leagues with the X850XT at half the price (online USA prices anyway). I also don't know how they perform in your game either, but wanted to comment on pricing/availability. $200 is too much money for a X850XT unless it's a HIS Ice Q II with the nice cool/quiet cooler and you want the game flatout. So at $200/$300 I'd grab the 7900GT before it's gone. At $160 and $330, I'd be considering if my games really need more than the X850XT.