#1 Did you have an Nvidia card in the box before you installed the Radeon, and if so did you COMPLETELY (i.e reformat)remove all the nvidia drivers? This is a known issue that severly effects performance of ATI cards. I'd be willing to be that this is the issue you are experiencing.
#2 What options do you have turned off/on in the BIOS, (specificaly AGP aperture size, fastwrites on/off etc.