if your looking at just the 8800 series of cards, it should be in this order, from least amount of "power" to greatest...
GS <--are you serious?
GTS <--meh
GT <-- good
GTX <-- great
GTX 280 <--now we're talking...
Also, for the GT, stick to the 512 mb version as it is an upgrade over the 320 and 640 mb versions.
GTS 640mb is not that win :(