There is one thing that will take you forever to work out if it's the problem and you don't know about it.
This is a long shot but a possibility, well worth trying to safe you some grief if it's the prob.
Try moving your PCI addon cards away from the end PCI slots (leave slot 4 or 5 empty if you have a five PCI slot motherboard) on your motherboard.
I don't know why this is, but sometimes the arrangement of cards in PCI slots can cause havoc in my experience.
Also, you could try reinstalling direct X, sometimes files will corrupt. with Direct X , if it asks for a reboot, then it has found missing or corrupt files.
After that, reinstall your motherboard AGP driver and check the BIOS setting for 4X AGP. Also set the graphics window or aperature to half your system ram. And lastly in BIOS, check the Primary VGA Card setting is for AGP Graphics Card.
As for Nvidia Drivers, go through the advanced options and tweak if you havent already, there are guides on how to do this, try
www.guru3d.com for help.
good luck
Mainboard: Asus P5K-Premium, CPU=Intel E6850 @ x8x450fsb 3.6ghz, RAM: 4gb PC8500 Team Dark, Video: NV8800GT, HDD: 2x1Tb Samsung F3 RAID-0 + 1Tb F3, PSU: Antec 550 Basiq, OS: Win7x64, Display: 24&