I was thinking of doing something very simular (but I'd have to use new parts as I haven't got access to 2nd hand). But I saw a cheap shuttle design thet took Socket 370 and had six channel sound, tv out etc. and a celeron 1.4Ghz. (£170 inc VAT + Delivery) Should give it enough zip. I've been looking for a settop type box to play DVDs, MP3 and Digital TV solution and emulate Amiga stuff. Seemed quite good only I heard the Shuttles make a lot of noise so wouldn't be a good under my TV!
Now I'm interested in iTX. And I might not have to get a PC box after all...roll on the Amiga One Lite!