XBMC runs on a wide range of platforms and hardware levels. The minimum requirements have more to do with your expectations and source content.
Playing 1080p or using resource-intensive skins will be problematic on low-power systems, but generally XBMC will run well on any system capable of Vista or even XP in some cases.
Best bet, run it and see how it performs for you. Then people can help you decide where to upgrade if required
System: XBMC HTPC with HDMI WASAPI & AudioEngine - Denon AVR-3808CI - Denon DVD-5900 Universal Player - Denon DCM-27 CD-Changer
- Sony BDP-S580 Blu-Ray - X-Box 360 - Android tablet wireless remote - 7.1 Streem/Axiom/Velodyne Surround System If I have been able to help feel free to add to my reputation +/- below - thanks!