A non-Apple card would need to be a flashed card. And no, G5 cards are not the cards you are looking for as they are all too new for accel under OS 9.

An Apple GeForce 4 Ti is what you want if you want the fastest OS 9 card. They actually shipped in QuickSilver 2002 and MDD models(IIRC), by the way. and a GF 4MX is slower than a GF 4Ti.