I just built a new Mini-ITX HTPC to replace the current one in my signature. It's running AMD's E350 series and it handles XBMC great, runs 1080p content no problem with DVXA2 acceleration, produces hardly any heat and barely uses any power to run. Not to mention it's TINY.
I paired 4 gigs of ram with it and a case for $200. Plus I was I was able to stick a OS hdd along with 2x3TB drives and a 1.5TB in it. I highly recommend checking them out.
EDIT: If people are intersted, I can post some pics online to show the set up. It's pretty slick for how small it is.