Depends what resolution you'll be playing at. Higher ones definitely a 7900GT. Lower resolutions, however, the 7900GS seems to be a winner. Plus you'd save about $100.
More like $40-50 savings tops...With the 7900gs in stock @ Newegg at $190-210 (some higher still)...And 7900gt between $230-260 (AR on the low end as well). And if you're willing to roll the dice with some of the eVGA recerts (the older models with possibly BAD Samsung ICs), you can get a 7900gt for the same price as the gs..Albeit with only a 90 day warranty (as compared to their standard 'lifetime' which coers cooler swaps and overclocking.
Personally, I'd go for one of the 'Reload' eVGA cards (gt) and pocket the $30-50 left over, or put that towards another HD, or maybe memory upgrade if you're @ 512MB-1GB.