I appreciate your noting your independent corroboration here - there are some people who are not prepared to accept some motherboards could have a problem with the PCI VDPAU cards. (To be clear this comment is not directed at NVIDIA)
Can you easily identify the chipset(s) involved here? The GA 7VRXP uses KT333, which interestingly is not listed as a supported "AGP" chipset at ftp://download.nvidia.com/XFree86/Li...hapter-12.html
It will be interesting to see if your chipset is listed there. (I'm happy to check if you do not have the information readily at hand)
Also another commonality is that Athlon XP CPU's feature in our experience.
The GA 7VRXP gigabyte manual (easily downloadable) has a "VGA Boot From" BIOS option where PCI or AGP can be selected.
Also possibly related is http://www.mythtv.org/wiki/NVidiaProprietaryDriver
(search for KT333) where reducing the AGP speed may be worth trying however I've abandoned those motherboards and have some other priorities before I might be inclined to reassemble one and try that.
Finally recycling P3-500's into HD frontends using PCI cards seems a good use of that old kit, if the UI speed is acceptable? Well done, I didn't think a P3-500 would have enough puff! I wonder if its chipset features as compatible on the 190.42 reference? I note the supported chipsets go back a long way in CPU history!