This was something I researched extensively already. Once you get down to it, it's a limited batch that can actually run in these systems. Firstly the card needs to have a programmable ROM, then it needs to be able to accommodate the larger Mac ROM, then it actually needs to have the proper flash tools written for it.

Best bet is with the 64mb GeForce2 MX and ATI Radeon product lines, plus a handful of newer ones. All the GeForce2 MX cards i've owned haven't been flashable, unfortunately.