Chec out sites like Tom's hardware or simular pages.
http://www.tomshardware.com/
Judging from the 9800 and that fact you want to get a 7800GS, you have an apg board. I strongly suggest that if you have the money you could possibly upgrade to a new motherboard with pci express. The 7800GS looks like its going to be the last card made in apg.
I got my hands on a 7800GT a month ago and built a new computer to run it for $800 because my old box was apg. Used some of the things fro my old computer to keep the price down. Specs: 7800GT, AMD Athhlon 64 X2 4200, gig ddr2 6 something, msi k9 platnum, x-fi sound card, 2 NEC DVD burners, 2 250 gig WD hd.
EDIT: Saw the post before mine - I think I read somewhere that the GS cards are ones that failed or just weren't quite good enough in the tests to become GT's because the GS's are toned down because of the apg's lower bus rates. (don't hold me to that though)