Hmm, well actually, the motherboard's part number doesn't tell me anything. What's important is the chipset.
However, it seems from the message "unsupported bridge" that your chipset either isn't recognized, or doesn't have any code written for it. If this is an nForce board, then that's probably the problem.
You say you're running a custom 2.4.19 -- when you configured it, did you configure agpgart as a module? If not, then right after the floppy checks is actually the right place (well... more or less) for this kind of message. Even if you built it as a module, it could be that the kernel is just probing for what PCI devices it understands at that point (PCI->AGP bridge chips are PCI devices on bus 0), and the fact that it doesn't understand that device could be passed down to the agpgart.o module anyway.
If agpgart is a module, does adding options agpgart agp_try_unsupported=1 to /etc/modules.conf and running /sbin/depmod -a help at all?