Is the Windows 7 Premium an upgrade? Because MS announced recently that you can use the RC to upgrade. But after upgrade you need to make a backup image using the great new Windows 7 Backup imaging app. so that you will never have to reinstall, because if you do then you'll need to reinstall XP/Vista to reinstall the upgrade after the RC expires.
An upgrade can be installed to another primary formatted partition or even another primary formatted hard drive, but the upgraded OS is then defunct according to the license agreement except to do a reinstall of the upgrade.
I am going to do my Upgrades to a second hard drive, then remove the XP/Vista/RC hard drive I upgraded from and plug the second drive into it's cable (set to boot). I'll have to run startup repair on the remaining Upgrade HDD, but I'll have a formatted clean install.
If you are asking whether you can install retail Windows 7 Premium over the RC, to get the cleanest install you should boot from the disk and do a custom install using Advanced tools to delete and then create a partition which you then format.