I would think a G4 tower with AGP would be a better OSX machine, leaving the B&W to be a super fast OS9 machine. A rage128 isn't that bad on a B&W in the 66mhz slot either (I have a few that have been relegated to PCI era machines).

PCI Radeon Mac editions are only going to get so cheap because few were made and people still want them (secondary monitor for G4's for example, or for PCI macs). Take a look at 3dfx voodoo 5500 PCI cards, they cost way too much even today.