Actually, a fairly common problem with this configuration.
This mainboard has the VIA chipset, so it's important to get the latest 4-in-1 drivers from viaarena.com. Direct link to the latest Hyperion Drivers:
http://www.viaarena.com/Driver/VIA_HyperionPro_V506A.zip
You should also have the latest BIOS from
www.asus.com for your mainboard flashed/installed.
After this, the things you can do:
1) Reduce AGP speed to 4x in SMARTGART from the ATI drivers or your BIOS.
2) Disable FastWrites in SMARTGART in the drivers, or your BIOS.
3) Disable VPU Recovery in the ATI Drivers.
As you've been trying different drivers, I'd also highly recommend getting Driver Cleaner Pro at
www.drivercleaner.net. You run this in Safe Mode as Adminstrator with the network/broadband disconnected right before installing new ATI drivers. The ATI uninstallers do not always get all traces or registry entries clean!
If these above do not solve your problem, hit ATI's site or Guru3d and look for the "Older Drivers" and see if you can find the 5.2 drivers. These solve the problem in a great number of P4V8X chipset mainboards.
memtest86 is also recommended as this can find any possible memory issues or misconfigurations easily. If it can get through a few passes of this test, it adds assurance that your memory is compatible with your mainboard. Let me tell you though- these PM800 chipsets are very, very finicky about what memory you run and their settings!
Good luck!