The PowerMac G3 should take most Macintosh PCI cards. The PowerMac G3 does not have AGP (which I learned recently) so the only way to go is PCI, and be sure to see if the video card will work with your operating system if your using Mac OS X. Although at the price of this graphics card I'm not sure if it's worth it.
I've seen some Mac PCI video cards on eBay for around $30, although some of those are PC cards flashed to work on Macs, although those don't always work well (if at all) you may be lucky. But anyway just be careful what you buy.