Just doing a search for "htpc" will get you into a whole evening's worth of diy fantasies. Here are a couple of mine:
http://www.cappuccinopc.com/dg1.asp - I kept this under $900.00 with 160GB drive and Windows XP Home. Don't even need XP home if you're savvy enough to take linux on. Or, if you really want redundant RAID, go with a custom case like this.

Funny how it seems expensive if it's an audio component, but cheap if it's a computer, eh?

Drool.