You can try for a pci GT520 is you can track one down beyond that you will be stuck looking for a older agp card and using the DVI to HDMI adapter. 7950gt agp has HDCP support for HD content but any thing better that is easier to find will be a 4650/70 agp from ati.
My P3 retro box, 80gb boot drive, 512mb ram, 1.4ghz p3, bfg 7800gs, bfg ageia physx ppu, and a upgraded psu.