>=4GB ram=3d display problems with nvida based video card

Good Day
After upgrading my system from 2GB to 4GB of ram, I am having problems
with applications that display anything in 3d. This includes unreal
tournament as well as the built-in vista "media center" and aero. I do
not run with aero enabled, but tried it just as a test. After a reboot,
I can load an application that uses 3d once, and it will work fine, game
plays great, or I can watch a dvd. Upon trying to load a 3d application
again, the frame rate is abysmal, < 1 frame per second. This then also
affects 2d apps, they do not refresh areas of the screen in a timely
fashion. This happens with both the builtin vista drivers for my
graphics card and the latest 101.41 from nvidia. Dxdiag reports that my
video card has 487mb approx total memory, when it actually has 128mb
onboard, and the agp aperture size is also set to 128mb Other than this,
the system runs great. I am thinking that this is a driver issue, but
was wondering if anyone else had a similar experience or know of a work
around.
System specs:
Tyan 2875 m/b
2 opteron 246
4 gb ram
Gigabyte 6800 based agp video card w/128mb
Windows vista ultimate x64
Thank you for any and all replies