I've been running a HTPC for several years, gone through several hardware/software iterations, w/ the media files store locally as well as in a NAS.
First, XBMC is excellent. Good choice. It supports a network database seamlessly -- along with locally stored files if you want to split your media library for whatever reason. There is little or no user-perceivable difference between having the files on local drives vs a NAS -- assuming the NAS and network have decent performance.
If you go with a NAS, gigabit network is preferable. 100mbit works but can sometimes result it hitches/glitches, esp if you have multiple users on the network.
Even though there seems little need for a high performance CPU, it becomes very desirable as the media file database gets bigger. I have 20,000 songs, thousands of movie/tv/music video files -- browsing through that database with an Atom is doable but slow. Actually running a quadcore 3.3Ghz Athlon II 640 right now. Smooth, enough power for anything I do with XBMC, browsers, etc. Preferable to a dualcore 2.4Ghz Athlon I had in there before and way better than a recent Atom dualcore I tried briefly. The latter often had screen glitches whenever I stopped/ff/rw any HD video. Power consumption w/ 1TB notebook drive & 80+ Gold PSU is ~40W idle, 50s on HD play. It could be a bit lower with a Sandy/Ivy Bridge CPU, but it's low enough; as soon as the big plasma gets turned on, we're looking at ~200W anyway, so saving 5-10W here seems trivial.
The noise of 6 HDDs in a PC in the same room would be a bit much for me. But it might be OK for you. Have a look at the DIY SFF systems described in our massive Home Server build guide: http://www.silentpcreview.com/Silent_Ho ... uild_Guide
I am using a HP Microserver w/ 4 2TB WD Greens as a home server in a closet on another floor. Even with its fan swapped and slowed & the whole system sitting on a block of foam, it's not that quiet, but outside that closet, completely inaudible. It runs 24/7. It could easily be a DIY atom-based SFF PC.
I don't use RAID -- just a PITA, imo. I do use other PCs on the network to run backups of folders I consider important -- mostly the music files, as I rarely want to watch a movie more than once, even great ones, unless a decade has passed since the first viewing. This is good because even in mostly FLAC format, those 20,000 songs only take up <500GB, a trivial amount in these days of 4TB discs!