7800GS cards are still going for 300$. That is totally unjustifiable when 7900GT's cost less now and are so much faster (especially if ocing is taken into consideration). What cpu do you have? Unless it's a socket 478 P4 or socket A AMD, it makes more sense for you to get a a pci-e mobo and something like a X1800XT (going for 220ish after MIR atm) than a 7800GS.
X800/850's, however, are cheap enough to justify staying with agp. For 300ish you could upgrade to pci-e/x1800XT (depending on your cpu of course) or for 150ish you could snag a used X850XT (or maybe even new, haven't checked the prices).