I've got an HTPC, roughly equivalent to my desktop.
It's got an ATX case, but a fairly tiny one so it's almost matx size.
Triple core phenom at 2.5ghz, 1 250GB ide hard drive and 2 750GB WD Green drives, a blu-ray drive (I think it's a burner as well), 4GB of ram, 64 bit vista (for the games!), wireless G card, runs Boxee (fantastic media center program), hooked up via HDMI to a 1080P display, and runs a DAAP server to share music over the network. I figured out how to set up DAAP before UPNP, plus DAAP is better supported by Linux and everyone has itunes anyway.
I've got the PC tucked away in a closet and just run the HDMI cable out. When I move, I'll have to see how I set it up in the new place.
Has an xbox 360 controller, a wireless gyroscope mouse, and a wireless keyboard.