My HTPC had an Ion, and I had flawless hardware decoding of 1080p H.264 in XBMC on Linux about 2 years ago. Unfortunately, I can't give you an easy step-by-step guide, because I spent like a week tweaking the system, but it was basically a then-recent build of XBMC with hardware acceleration provided by VDPAU. It was an amazing setup. (Everything is in the past tense because I've since moved to San Francisco and don't have room for a TV.)
http://www.newegg.com/Product/Product.aspx?Item=N82E16856119...
http://en.wikipedia.org/wiki/VDPAU