|you can get decent (and up-to-date) motherboards and processors for the price of an agp card with video out.|
why waste your time with this, even if it could be done, the picture quality would render your hdtv useless, and i doubt you'll find any agp or pci cards with hdmi connections.
my advice, look in your local paper for someone selling second hand pc or laptop, or google reconditioned motherboards, processors, graphics etc
Windows 7 x64
Intel® Core™2 Duo CPU E8500 @ 4.1GHz
ATI Radeon HD4870 XFire
8GB OCZ Platinum 1066
2x Samsung 1TB Spinpoint F1