First thing what ever you do
dont go and pay for one of these patchsticks you see advetised all over the site.
Go to this site and download the atvusb-creator for either Windows or Mac.
http://code.google.com/p/atvusb-creator/
I've tried XBMC on my AppleTV several times but have never been completely happy with it's performance, the user interface just seems too sluggish on the AppleTV and it's hit and miss as to weather videos play smoothly. Coming from an xbox you might think it's an improvement.
If all you want the AppleTV for is XBMC and nothing else then you may be better off with one of the small Atom PC's like the ASRock 330 Ion or the Revo. They run XBMC a lot better and with hardware acceleration right upto 1080p. I've got a ASRock 330 Ion running XBMC and it performs much better that the AppleTV and costs about the same. Althrough it's not as easy to insatll and set up, but if you managed it on a xbox you should have no problems.