Have you already purchased your Zotac Ion board?
Honestly, I had replaced my AMD HTPC (5050e dualcore + ATI HD4350) with a Ion 330 system and recently I reverted back to the AMD system: sure, dvd, HD and bluray playback is doable and even better than decent, but in the end I just wasn't satisfied with the Ion platform.
And having laggy or choppy flash performance just became annoying.
If you DO have the Ion platform, give this
Ion optmized distro of XBMC a look.
The Ion platform does make a great ubuntu server system though...!