I guess wat it boils down to is what do you have the money for and how many slots do you have open on your machine... they are bacisally the same card... they're both 512MB, but the GTX has a higher core/clock speed... but if your not really going to OC then what difference does it make... at tat speed you visually won't see a change anyway...
if you plan on overcloking that 7900gt then you'll prolly need a new heatsinkfan and its not guaranteed to go to the 7900gtx.. i'd just go for the 7900gtx it comes with a much better hsf and its preoverlocked to 7900gtx :P
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius