Save up for a whole new system. Your P4 would choke pretty much any new graphics card you put in there. I would not bother with trying to find an AGP part as the same is true of the best AGP cards too.
Agreed with above. To answer your question I'm not sure there are any. S775 arrived about the same time PCIe did. The first Intel chipsets that supported PCIe were the 915/925. These were S775 only AFAIK. Its possible that Asrock made some hybrid board, but they would be rare and very expensive.