Nice card ;o)
Does your board have an onboard graphics option? Can you get inside and remove the 8800? Are you willing to? Others may have other ideas, but I had trouble too (see below).
I tried installing Vista Ultimate with my new card (Radeon X1650 Pro 512MB) in place and the install just wouldn't work. After leaving on 'Completing Installation' (on the odd occasion it got that far) for 14 hours (!!!) I took the card out, put my old one in and the install worked flawlessly in well under 30 minutes. I then put the new card in and it's been fine since.
I don't know whether it was a driver issue or a power drain (my card needed to be connected to the PSU) but the install was not for having it with the powerful card.
Your inital install, especially with onboard graphics, will not have the Aero interface etc but that should correct itself when you put the 8800 back in. Vista just makes the Aero Experience unavailable to select if your PC can't handle it.