Yeah, nVidia isn't exactly supporting the 7950GX2 anymore so newer drivers tend to give it more bugs than they fix. Use drive cleaner to remove any traces of your current driver and try using an older driver version that you know works.
Unfortunately, this is a new card, on a new motherboard and power supply. I've never used an Optiplex or a GX2 before, so this is new to me.
These cards seem to be a real pain to install on a system, and it doesn't help that this computer didn't come with any drivers.
Also, the is there any way to tell if a graphics card is working? The system seems to be detecting it.
By the way, here are detailed specs for my system:
CPU: Intel Pentium 4 3.20 GHz single core
Motherboard: Intel i915P/i915G PCI Express x16
Graphics card: NVIDIA GeForce 7950 GX2 512MB or integrated Intel 82915G/GV/910GL Express Chipset Family
Sound Card: SoundMAX Integrated Digital Audio
CD Drive: SONY CD-RW CRX216E
Power Supply: OCZ 600W
Operating System: Windows XP SP2