Originally Posted by iranu
The x1800xt is a faster card than the 7800gtx in the majority of games and cope a bit better with AA/AF. The drivers are equal in my opinion, ie there is adequate support for both cards from their manufacturers.
Therefore it is really a question of budget. Find the cheapest price for both cards and evaluate extra cost v performance gain. Remember what resolution you want to run at and the level of AA/AF.
If the prices are the same I would go for the x1800xt. Either way you shouldn't be dissapointed, they are both fast cards capable of running all current games well.