I think I'm going to build. I'm probably going with an Oregin AE case, Quad Core MB, 4 Gigs RAM, 1 80 Gig HD for Vista, and 1 TB drive for storage. I'm thinking about using a Creative Labs Sound Card, and I'm not sure about the video card yet though it's not super critical since I won't be doing blu-ray or gaming. I'm thinking about using 3 tuner cards. I'll access everything with a Logitech dnova keyboard.
Honestly, this will be primarily a music server at first, so it's probably way overkill, but I'll eventually incorporate more and more of the HTPC functionality as time goes on. I no longer have the time or patience to constantly go and take things apart and upgrade, so I'm gonna try to build something a bit future-proof... though that sounds a bit naive when I think about it... but once I build it, I want to leave it for awhile.