I've currently got a media PC running Windows 7 on an old Athlon XP 2600+, which clocks at 2.1GHz, with 2GB DDR1 (not sure what speed) and a Radeon 9600 Pro 512MB. Basically an old gaming rig. It still runs Windows 7 fairly decently, given the age of the hardware, but XBMC has some graphical lag...