I having the same problem with you. The only difference is mine is a HD 5830. Still looking for answers.
I've tried changing the settings in the BIOS but it's not working. I have also uninstalled all the other drivers but nothing seems to work.I borrowed a CPU from my friend (an i3 running on a PCI-e 2.0) and plugged in the HD 5830. Fired up without a hitch! Just to make sure my PSU is working fine, I switched the PSU and no problem at all! Oh no! Don't tell me I need to upgrade my motherboard, CPU and RAM just to use the HD 5830!!!!! God! No!!!!!