yea, reformat ur hd, it will only take 1 hour max. Its best way to get it to work. But try this 1st.
take off any ati drivers u have. put in ur old video card. then, go to deveice manager, and select ur display adapter, and remove it.
then, make shur all drivers are uninstalled. then turn pc off, and swap the vid cards. turn pc on, and install the latest ati drivers 4.8, dont install the 4.9 beta, just to get it to work, then if u get it working, u can switch to he beta doom 3 drivers.
Hope this helped
My current specs:
Biostar TPower x58
XFI GTX260 Black Edition
640 WD Caviar Black
Vista Home Premium x64