An X-BOX1 (not 360) soft-mod to run XBMC + HDTV component cable.
Cost = £30
It is the cheapest, and IMHO the best HTPC you can get.
It's getting a little long in the tooth and yes in 1080i modes it's 733MHz CPU and 64Mbytes of RAM make it a little slow but in 720p it's still excellent.
XBMC is without a doubt the most polished media player available for any platform.
Plus you've got the entire back catalogue of XBOX-1 games to downlo.. erm buy...I mean buy. ;-)
And a whole host of quality Emulators including WinUAE-X
