Just thought I would put my two cents in on this as I have had a HTPC in my theater for almost two years. What I have found is that my Windows Home Server, WHS, is a better solution. I store little media content on my HTPC and instead store everything on my WHS. I access my media files from the WHS server on any device that is attached to my home network this includes my HTPC. I can access everything I own via the web as well.
My networked HTPC, BluRay players, PS3, Denon & Onkyo AVR's, and Sony TV hook in and play everything I have stored on it seemlessly. All 9 of my PC's and laptops are backed up daily and all media files my family places on them are copied to it which then are shared from one location. All virus protection is distributed and managed from it. It runs 24/7 and when built correctly requires very little hardware investment.
I think that Windows Home Server, WHS, is truly the least understood and one of the best products Microsoft has ever released. it is the best backbone for any home network. It just runs and does it's job with little fuss.
Check out HP SmartMedia Servers. I own the first gen EX470 which I modified with a dual core cpu and memory upgrade. Very cheap but powerful solution.
Lifes a beach and then you die....