to add to what
@bunnspecial has said
the Radeon 9200 Mac edition is also probably one of the easer to find high end official Mac edition cards for a BW G3 there is usually always 1 or 2 on ebay for a good price (proper Mac edition Radeon 7000s are actually much harder to track down but they do show up from time to time but are nowhere near the best card for a BW G3) the Radeon 9200 will also work fully in Mac OS 9 and in BW G3 without a G4 CPU
if you have a G4 then flashing a FX 5200 is a good cheap option which gets you partial core image as well, and there is one type of PCI GeForce 6200 that can be flashed, but the later requires a G4 CPU but gets you full core image but if you have a lot of high bandwidth PCI cards installed can cause the PCI bus to throw a wobbly (the PCI layout of a BW G3 sucks), the FX 5200 will work with a G3 CPU however some 3d apps may cause the GeForce kexts (and consequently the machine) to crash without a G4 hence why I recommend a G4 if your using a FX 5200 however its not required. (the GeForce 6200 requires a G4 otherwise OS X wont boot, I flashed one here
https://forums.macrumors.com/threads/some-ppc-openmark-benchmarks.1848060/page-6#post-24270500 )
all in all if you want a nice OOB experience with the most OS support I would highly recommend a Radeon 9200 Mac edition (also have you checked the RAM on the BW G3? I found mine being a Rev A machine is VERY touchy when it comes to wanting to POST properly and usually ill just have to move the ram sticks around and then ill get a picture, also as long as none of the pins on the Rage 128 are touching each other then it should be fine, those pins on the card it self are for an optional DVD decoder card)
