You can buy a NVidia GeForce 6600-6800, or ATI X-series, but your system is too old/slow to deal with any new AGP card (by new I mean more or less modern chip).
I have had this PC for 2 years a long time ago.
The major issues of that model are:
- SiS 661FX chipset used on its P5S800-VM mobo
- Pentium 4 519J CPU
both are bottle necks for a more powerful VGA
I've upgraded my PC with ASUS P5P800
and Pentium 4 531 (with HT and extra 512 Kb of L2)
after that I've noticed a performance leap, so I decided to buy an ATI X1950 Pro AGPx8
But even with new mobo and CPU, my VGA could not work at 100%.
DDR-1 is pretty slow (even if I oc'ed it from original 400 to 475 Mhz)
P.S. Buy a new cheap PC, even if it will be based on Intel Atom CPU you will gain in performance.