The MacPro, with PCI-express slots, does not have an AGP slot, so nogo for any AGP card, PC or otherwise. Depending on the G5 PowerMac, most have an 8x AGP slot. The last G5 also does not have an AGP slot, moving to PCI Express slots for that model.
An AGP PC video card needs to be flashed to a Mac ROM, usually while installed in a PC. The resulting hybrid video card is often unstable, although some will work. Depends on the brand/type of vid card. xlr8yourmac.com is a good site to check for possible PC video cards that can possibly be made to work in a Mac.