Just assembled my own Windows XP Media Center computer to replace my old DVD players etc - this was fun and what an experience it is in our living room right now. We can watch our photos, videos and listen to music coordinate it from one common place, this post is written on the MCE and plasma screen. Watching the WMV High Def demos is really awesome!
Heres what I assembled and connected to my Pioneer PDP-435:
Asus motherboard with a Intel P4 3.2GHz HT, 2GB RAM inside a Ahanix case, with a lot of space for more harddrives. A Hauppage Win PVR-500 (dual tuners) video card and some other stuff. An Asus graphics card with Nvidia GeForce 6600 GPU(fan free).
I put in a cheap DVD-RW (HD-DVD or Blue-ray will replace this as soon as possible) and made it silent with Nero DriveSpeed - when playing DVDs it's not necessary to have it spin 16x and sound like a bulldozer!
For right now it sounds a bit to much, but some new and silent fans is on the way and some sound proof padding for the case is needed. If thats not enough - then it will be water cooled :-)