I can verify that it is no problem. I had lots of games installed XP and installed a dual boot Vista (both 64bit). To play my games on Vista all I needed to do was install updates, service pack one, and directx 9. To get my save games to work I had to copy the files from docs and settings/user folder for xp to the user folder in vista. So basically install updates, directx 9 and make shortcuts to your game executables. No need to install game twice or play with the registry.
Only game I tried that didn't work right doing this was Gears of War, and I think it had something to do with Windows Live, but I don't remember.