That does depend on the card in question. The GTX is the highest performing version of the card, followed by the GT, then the GS (and there are many other abbreviations as well). Obviously, not all cards have versions with all three model numbers (e.g. there's a 7600GT, 7600GS, but no 7600GTX).
There are some other considerations, though. The 7800GS in particular is an AGP card whereas the 7800GT and 7800GTX are PCI-E. In the case of the 6800, the GT performs only very slightly better than the GS, but costs significantly more. I wouldn't buy either, though, the 7600GT is much better.