This is pretty much the deal in the $300 range.
- X1800XT at stock beats a 7900GT at stock.
- The 7900GT has shown to be a massive overclocker when volt modded for quite a few people
- The X1800XT is no overclocking slouch either, but doesn't easily clock as high as the 7900GT's
- ATI is phasing the X1800XT out, so you'll need to get one quick should you decide to go this route.
- The X1800XT runs hotter and consumes more power than the 7900GT.
- The fans on both aren't the quietest fans out there, the X1800XT is probably a bit louder or more noticeable.
- The X1800XT supports HDR+AA and has overall better image quality, although neither of them have poor IQ.
That's pretty much what I have gotten out of the numerous threads on the subject and my personal experience. These two cards are very evenly matched, so the decision isn't easy. Personally, if I was in your shoes and I could find an X1800XT with 512MB (there are 256MB and 512MB versions), I woud get that. There are already a few games that benefit from 512MB of video RAM, and that number is only going to increase.