HTPC Need advice on spec for new HTPC

My flatmate is leaving in september and taking with him the 80's VHS recorderesque media pc we've been using for the last year. This necesitates me building a new one of my own, as i previously used an xbox with xbmc, but have since upgraded to a 720p TV.

I'd like to use the antec fusion case, and I'd like to be able to playback 1080p x264/h264 files for when i inevitably upgrade the display. My media is stored on a file server in a cupboard and we currently use XBMC on Ubuntu for the media box, which I'd like to stick to. I'm also not too bothered about having a blueray or hd-dvd drive.

Does anyone have any experience of what kind of hardware spec I'll need? I was looking at microatx boards and the amd be-2350 and similar chips, but i'm not sure of the support for hardware acceleration under linux with some of the newer onboard chipsets which are meant to be good. i plan on using a small 2.5" HD and keeping cost down generally, so i need to get enough performance out of onboard sound and video, preferably with an hdmi port. Any advice?