To my knowledge you won't need to install XP, you can use the Win7 RC or RTM disk to to a clean install. Just keep a few things in mind.
1) If you have the beta (build 7000) as opposed to the RC (7100) or the RTM (7600), remember that it has already hit the auto-shutdown phase. Your PC will turn itself off every 2 hours if you use the beta.
2) You cannot upgrade XP or any of the pre-release editions of Windows 7 to the full version, it has to be a clean install.