I tried to use a Atom/Ion (Zotac Zbox HD-11) running Ubuntu and I just cannot get HD video to render smoothly. Very frustrating. There is no substitute for processing power. If the machine doesn't do what you want, the fact that it's only sucking 20W doesn't really matter.
Personally, I think RAID is overrated. RAID is NOT Backup. Just get a nice, big, fast hard disk and back it up to another machine on your network and/or an external drive "often".
I'm thinking an Intel i3 for a budget HTPC build.
BUT...
I've had a PC in the living room for years, now. And I'm kind of "over it". I think the industry is headed more towards streaming boxes like the WD, boxee, Roku, etc. We're using iPads to control the Squeezebox touch for music streaming. That totally solved the "access to music" problem. It also gives me internet in my lap (where I can see it) but not on the big screen (where I really can't).
I wish you a joyful journey and satisfying technology experiences.
