I've heard that apple's opengl implementation is altivec (g4) enhanced. Therefore I'd bet a slower (mhz wise) g4 could keep up w/ this upgrade card. Plus, most g4's have agp slots for graphics cards while the blue and white g3 only has pci.
Can a game use your graphics card in osX if it is below a 32 meg card? Or does the 32 meg card only for quartz extreme?