Well, I made my decision. I'm killing my desktop.
I swapped Thor's Athlon 64 mobo with Manfred's Athlon XP motherboard under the assumption that the 64bit stuff will be put to more use in the PVR. That went fine (Thor was still FC4 - i386 version :-/). The problem came when I looked at the Athlon 64 motherboard and realized that it's PCI-Express. And I don't have any video cards that have a TV-Out that isn't AGP.
So I think I'm going to buy a X300-based PCI Express video card (about \$35 on NewEgg). At that point my PVR box should be functional. I'll just have to live until then (maybe next weekend I can finish it).