I have a similar system. There have been a ton suggestions and reasons for all the troubles. The fact is the 9700 8X AGP doesn't work well with some systems ( not just the Granite Bay CS). I was able get mine working by doing the following:
Unistall your old ATI drivers ( and make sure any Nvida is gone also), set APG voltage to 1.7v, turn off fast writes, and set AGP apteture memory to 128, and make sure your using Catalyst 3.2 .
3.3s should be out April 29th... don't get the early version out now they are OEM targeted for the Dells with the 9800s.
If all else fails, drop down your AGP to 4X with fast writes off.

I know, it sux to run at 4X with a new system that's supposed to go to 8X. But alot of folks run stable with 4X .
Good Luck!