I believe the cards are rendering in the old PowerPC Macs like the G4 quicksilver and MDD, to a point anyway. I believe where the big changes come are in G5 and up, then the graphics cards become less important... at least that's what I've been reading.
The problem with the G4's is that they will only read up to 128mb of the VRAM anyway, so even if I were to flash a 256mb card to put in there, it will still limit to the 128mb. Otherwise, I'd be more adventurous and flash a better card. The Radeon 9000 Pro (stock in the MDD DP Macs) is supposed to be a pretty good card, not a GeForce 4 Ti, but still pretty damn good... so I might not see much improvment at all.
If I'm successful, I'll post the results... if not, I'm stayin quiet