I have the same exact problem with an ATI HD 2600 XT.

My specs:

Asrock 939Dual
AMD X2 3800
2GB of DDR

I tried swapping the video card for an older Radeon 9500 and that allowed me to get around the purple screen. Unfortunately, I ran into other problems once it came up with the 9500. So, it looks like the 2600 XT might be the problem.