You should be doing better than that. I play Doom3, UT2004 at 1024 x 768, high settings (doom), highest settings (UT2004) without problems.
If your power supply is good, then Im thinking driver conflict somewhere. Maybe reload latest Nvidia drivers (61.77) and use driver cleaner.
Before that, do the usual checks. Make sure VSync is off, AA and AF are off, set agp aperture in mobo bios to 256 or 512 (since you have lots of memory), turn off any WinXP services running in background that use memory (task manager and Auto updates are 2 biggies), turn off fastwrites, etc.
Post back how things go. Your system is like mine so you most definintly should be better. Also, can overclock your vid card and cpu to speed things up, but best to try and optimize your system first. Good luck