Hi Lee,
There's no reason if your hardware supports a 64-bit OS why it can't run Win 7 Pro. I assume custom installation really means clean installation.
As far as returning to Vista, apart from using the Dell recovery disc (if you prepared these), there's no way to go from a clean installation of any 64-bit OS back to the original OS without reinstalling. However, the upgrade advisor should tell you exactly which hardware and software needs to be updated. Honestly, I know of no one who ever went back to Vista after upgrading to Win 7. Virtually all hardware and software today are supported on both of those operating systems.