You get a little more for your money with a 6600GT compared to a 9800Pro.
And, in case you were wondering, the 6600GT will run with a 4xAGP slot.
FYI, If you find PC3200 cheaper than PC2100 (definite possibility), you can use PC3200 in your system, it will just run at PC2100 speeds.
Intel E6750...........PSN: ELBATROP
XFX nForce 650i Ultra
Patriot PC2-6400 8GB (4x2GB)
36GB WD Raptor
Samsung SyncMaster 931c
Samsung SyncMaster 750s
Windows 7 Home Premium 64