There are some issues when combining NF3 and 6xxx. I actually got the pleasure of experiencing the incompatibility myself. While gaming, I would get a slight pause in gameplay, which almost seemed like poor FPS, but wasn't.
With your mobo, go with an ATi card. The 9800 Pro is pretty comparable to the 6600GT. The x1600Pro I believe comes in AGP (which you need) as well.
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