From an MS' whitepaper:
Virtualization, a new benefit of Windows 7 Professional, Enterprise and Ultimate, helps small businesses upgrade to Windows 7 by providing a Windows XP Mode environment capable of running many Windows XP-compatible business and productivity applications. Using Windows Virtual PC, a new feature in Windows 7, customers can run many older Windows XP business and productivity applications within Windows XP Mode and launch them from the Windows 7 desktop
with just a single click.
Therefore, Windows XP Mode for Windows 7 gives you the best of both worlds: the ability to enjoy the many benefits of Windows 7—such as increased security, productivity, and reliability— while still getting the most from your Windows XP applications.
For Intels, disabling the 'green goodies' ("CPU Enhanced Halt (C1E)", "C2/C2E State Support", "C4/C4E State Support", "CPU EIST Function", and "C.I.A.2") is pretty much mandatory to OC - for AMDs, dunno that much...