Time for upgrade lol. I got an 8600 GT and it works great but i sure as hell can't push those new games on high, let alone ultra. Try turning down settings and play with the options. Shadows and antialiasing will be your enemy til ya get into the 9 series of the GeForce family.
Also try going into the NVidia control panel via right click on desktop and default everything and make sure the application always decides the 3d settings.