A 8800GT that performs better than some 8800GTSs, a 8800GTS 512Mb that performs better than a 8800GTS 640Mb and a 9800GTX that will probably have less performance than a 8800GTX.

nVidia is being a real "gentleman" for its less-informed costumers. Especially when it comes to erroneously naming it's graphics card in order to
a) emptying older cards' stocks (8800GT>8800GTS 320/640Mb)
b) getting it's clients to pay some extra $150 for a card that supposedly belongs to a new generation and in fact only has higher-clocked memory (8800GTS 512Mb = 9800GTX).