The FX 5600 is a good value right now, especially if you can find a used one. Since that particular card didn't work for you, you could try the FX 5600 Ultra with 128 MB. I just got that card for my system at home and G3 runs pretty well on it (450w power supply). I only spent about $115. Here at work I'm using the ATI 9700 on a Dell 4550 3.0 gig, which is a bit more expensive -- more than I would spend personally.

How many watts does your power supply put out?