I un-installed the nvidia drivers when I switched from a 780i to the X58 and I did not have to re-install Vista 64-bit. I didn't even need the Vista DVD. I did have to run the X58 driver CD, of course
And I did have to reactivate Vista online.
Of course you never know what may happen. As a precaution its always wise to remove all your drivers, video, sound, network, etc... And it doesn't hurt to remove/deactivate any game you play that has online activation requirements.
Edit: And have a Win7 disk made just in case.