The Q9550 should easily get upto 3ghz especially with an X48 chipset and probably on stock voltage.
Also before you buy the bits for your build I would check the Sudhian forums which has the biggest Shuttle forum on the internet AFAIK:
http://www.sudhian.com/index.php?/forums/
Shuttles are picky for RAM(as you may know already) and it maybe worth checking the forums for any known issues with the X48 chipset based Shuttle barebones.