You won't be able to do much better than a 3.6GHz P4 and X850XT. The best AGP card is the Gainward Bliss 7800GS which is actually a 512MB 24-pipe 7900GT. But it is not available in North America and costs a fortune, making it time to upgrade to PCI-e instead. The 16-pipe reference 7800GS is at best a bit above the X850XT, not worth the upgrade.
Best AGP money can buy:
Gainward BLISS GeForce 7800 GS+ SILENT 512MB GDDR3 TV-Out/Dual DVI (AGP) - Retail (GX-032-GW) http://www.overclockers.co.uk/acatalog/Nvidia_AGP_Graphics_Cards.html