You should to head over to AVS's HTPC forum and SPCR for all your quiet media servin' needs. I'm constantly amazed by how far us guys in the former have come in just a few years.

I'd avoid SFF PCs because very small and quiet are very hard to get in one package. Check SPCR to see what I mean. Their expandability also is almost nil - don't underestimate how big video files are. Even Shuttle's larger SFF PCs are limited. You also will need this space for futureproofing. Want to add DVR functionality? You'll need more drive capacity and an expansion slot for a DVR card. That sort of thing. Go for at least a midtower case if one box has to do it all. Antec makes good looking, high-quality ones if you can't hide the server. If you can hide it, say in a laundry room or in a basement, then I'd consider a big, cheap, old, noiser server with a silenced SFF HTPC serving as its client. The former isn't very expensive to build at all, as serving files is a very easy job even for an old PC. The latter will then not need much in the way of expandability. Of course, now you have to consider how to build your network. I opted for 802.11a wireless to my router so as to avoid interference in the 2.4GHz band. Wired networks are better, of course.

Avoid RAID if possible. RAID is for uptime, not backup. Even RAID 1 is not to be trusted. Slow degradation of a mirrored data drive means a perfect copy of corrupted data. Periodic backup is the way to go. Also, drives today are big enough that the convenience of RAID 5 (multiple drives appearing as one) is no longer relevant. Get a big drive for data and periodically back up to a second drive in a rack or its own enclosure.

For reference, my HTPC uses an Antec Sonata case with 4 hard drives, a removable drive rack for backup, and a DVD drive. Plus a network card, a DVR card, and a sound card. 120mm intake and exhaust fans, large Zalman heatsink/fan on the CPU, passively-cooled video card, that sort of thing. Very quiet and only mid-tower sized.