My dad picked up an Athlon 64 based HP with Media Center 2004 or 2005 (can't remember), but these are gone. Try http://www.geeks.com/products.asp?cat=SYS#Pentium4-HP and you see a few others from HP as well. These are likely the boxes with HP's DVD+RW drives and not DVD+/-RW drives and heavily discounted.
My dad is happy with this make/model and is having a great time.
I've been shopping for a new PC but I'm having a problem finding one with Windows Media Center already installed. I know Gateway has it but I can't deal with them. I'd prefer to buy from a in-store retailer, Best Buy, Circut City, etc...