R420 (used in X800 XT AGP), is a AGP native chip 😀
R423 (used in PCI-e version of X800 XT), is a native PCI-e chip.

Other example :
R481 and R480 should be used on X850 series Radeons depending on bus type (PCI-e, AGP).
R481 = AGP,

R480 = PCI-e.

In both cases bridges are not needed (unless you have to swap them around).

I suspect the card is bad. Maybe a solder failure.

yep I think it's a failed chip back to the fx 5900 I guess.