Have a good 4-8gb usb flash drive available? Brink just finished writing a great tutorial on how to install from a usb key rather then burning to a blank dvd if you just happen to lack a dvd burner. Often the older premade desktops as well as many laptops suffer in that area big time! USB Windows 7 Installation Key Drive - Create
Rather then the usual slap a leak on a dvd-r and toss a frisbies when found to be a fake or reformatting a dvd-rw for checking one out the flash drive here gets some use since you are able to start the setup.exe while in 7 as well as in Vista or XP! While the flash drive is still made bootable you don't run into one problem starting the installer while already in Windows.
You wouldn't have any problems starting the setup.exe file while booted in XP to start with being a previous version of Windows and seeing a custom install. The one thing you tend to still see is XP becoming the boot sector and not being mounted at first when booting into 7. Unfortunately that works in reverse of what you want making XP unavailable at first while in 7.
The one thing you will want to be familiar with effects 7 the same as with Vista namely the system restore cancelation problem when dual booting XP with 7. The Vista problem still remains seen in 7 as well where the method to protect 7's points is the same as Vista's. System Restore Points - Stop XP Dual Boot Delete - Vista Forums
The RC installer should simply pack all the 7000 folders into the Windows.old folder rather then trying to use the upgrade option. You would still plan on a fresh install all over again with the new registry. The RC will likely see far better upgrade capability while you still have to be prepared for a fresh Windows all over again when it replaces the 7000 install you have on now.