It is easy enough to find later revision G3 motherboards that trying to modify a Rev A one isn't worth the trouble if it can even be done. Try Olde Mac Milt on ebay.
For a G3, a Radeon 9200 would be good. You can get those with a Mac ROM without flashing. You can also get a Radeon 7000 with a Mac ROM if you have a smaller budget. If you plan on running OS X and need Core Image then an nVidia FX5200 flashed to Mac would be your best bet. Anything faster than that would be wasted because the bus can't pump data to the video card fast enough to keep it busy, so don't get too carried away upgrading the video because beyond a certain point you'll be wasting your money buying power your machine can't fully utilize.
For a G3, a Radeon 9200 would be good. You can get those with a Mac ROM without flashing. You can also get a Radeon 7000 with a Mac ROM if you have a smaller budget. If you plan on running OS X and need Core Image then an nVidia FX5200 flashed to Mac would be your best bet. Anything faster than that would be wasted because the bus can't pump data to the video card fast enough to keep it busy, so don't get too carried away upgrading the video because beyond a certain point you'll be wasting your money buying power your machine can't fully utilize.