Thanks for all the replies, guys.
Another question: Will I be able to use just an upgrade disc to 7 ultimate to go from the RC to the Ultimate?
If you are running the Windows 7 Release Candidate, and you have activated it, you will not need to reinstall an older version of Windows before using an Windows 7 upgrade disk.
However, you will not be able to do an upgrade installation, in which your existing programs and data are left in place. Instead, you'll have to do what Microsoft calls a custom installation.
This is essentially a clean install, but your existing operating system, programs and data are squirreled away in a folder labeled WINDOWS.OLD. You end up with a fresh Windows 7 setup, but you can access that folder to get to any needed data. (Sorry, the programs in there won't work - you'll need to reinstall them.)