You can do a clean install of windows 7 RC over windows 7 beta, all of your files(desktop / program files / drivers / other stuff) will be put into a folder called Windows.OLD. You can then navigate to that file and move things to the proper locations you want them at. There's no reason to back up data, as "clean install" won't reformat the harddrive.
As for settings, and program files you'll have to setup and reinstall them all over again (you can't simply move them over as the registry files attached to them won't work with the new system). Don't believe there is a way around that.