You could OC the lower one or UC the higher one. Even if you mix the revision with 96+ and the 96 shader version it should still work. HOWEVER if you have the G92 version I don't think it will work with the G80 version, just because it really is a totally different GPU with the same name.
Incase you weren't aware there are actually 3 different version of the 8800GTS. There's one with 96 shader processors and there is one with 96+ shader processors, which gets confusing because it has between 96 and 112 shaders(don't ask why). Then there is the G92 which a totally different bag of nuts, it's alot faster and like you were saying it has 512mb of ram and not 640.
Since you're considering another G80 GTS 640 I think you'll be ok.