I don't think there are any specific specs required unique to XBMC thats other software don't need. But I will try to give you some hints.
OS: Works on any OS, I prefer Linux, for 1 I don't need to buy a Windows License and 2, I can automate a few other things without the OS becoming slower on boot time.
Hardware spec.
GPU: Use something where the drivers support H.264 hardware assisted acceleration. Mostly important on Linux, on Windows any DirectX compatible card will work.
CPU: If your GPU is not going to assist for smooth video playback, at least 3 ghz dual core if its the Core2duo architecture, something similar if its AMD (I don't know their line that well). If your GPU will accelerate your playback anything from a Pentium4 or so will be good enough.
Memory: On Linux you can get away with 1gbyte memory, on Windows I personally would go with 2gbyte at least.
Hard Drives: Not big to get it functional, for practical reasons I am now at 1.5gbyte, but soon going to add 2 more 2gbyte drives.
If you want to give the XBMC Live a try (XBMC Linux Distro where most stuff is already set up for you):
http://wiki.xbmc.org/?title=XBMC_Live#XBMC_Live_minimum_and_recommended_hardware_requirements