Im running a 9800XT its a great card, but if your preparing for next gen games go with a x800 series.
I bought a 6800 GT OC and I havent looked back since. The thing runs Half Life 2, Doom 3, and Far Cry like a damned dream, and I wont have issues in performance aside from minor ram issues (my new motherboard does not like my ram), my computer runs just about anything with that GT on it.
you shud be lookng to next gen cards tbh, dont get AGP, get a new motherboard and proc with PCI-E SLI and go dual or even single. A 6800 AGP is VERY different to a 6800 PCI-E
If you just want to upgrade video then get the 9800 AGP, if you want more then youre gonna have to do some serious upgrades on the processor and motherboard, and I believe that Intel is the only company with full compatability with PCI express.
i got the sli twin ultras took alot of saving (no it didnt) and determenation (determenation = having a rich freind)
Definitly make sure you are building your comp with PCI-E, that is what the future will be with computers. A geforce 6800 256 mb is what I would suggest. PNY makes a nice one.