Those harbingers of hardware hacking over at ExtremeTech have writen a How To Article for building your own WIndows Media Center 2005 PC.
Their bottom line was $2203, but they made it HDTV capable, and used a pricey HTPC case. The most confusing aspect for me is that they used the much maligned (by them!) ATI HDTV Wonder. Apparently, WMCE 2005 does not support the analog NTSC portion of the HDTV Wonder, so they added the eVGA nVTV Hardware encoder card (I’d not heard of that card either.)
Check it out.