Want to build a Low Power/Small Form Factor NAS/HTPC

Ok so I may be trying to do too much.

But I've got a 3 year old, noisy desktop that I want to make into a HTPC/NAS. From that build I think I want to reuse the two WD 1TB Caviar Greens that I have, and am willing to buy whatever else I need.

Ideally this PC will be on all the time, accessible by the net, but should the need arise, function as a HTPC. The desktop now streams to the ipad/iphone and would like to keep that function if at all possible. Recording TV isn't necessary.

I'm in love with the Zotac Mini ITX NVIDIA ION mobos but not sure if that's what I really should be looking at, knowing that I've got at least two HDDs, probably a third for OS.

I'm also stuck looking for a case that would hold all this (plus most likely a BluRay or DVDRW).

Am I trying to do too much, and still have it energy efficient? Obviously a pico psu wouldn't be my best bet - neweggs calculator puts me needing 300W.

Am I doomed to have a noisy ass PC that kicks into high gear the minute Firefox seems to get a memory leak? Or an oversize desktop in the middle of my living room? Or are there other options that people have tried, and or built themselves?