According to the Macthis.org review of the developer preview version of OSX 10.2, there is a sizable improvement in screen draw performance even on lower than "recommended spec" graphic card systems. In this case the reviewer was testing on a dual processor 500MHz G4 with only an old ATI Rage 128 card (16MB).
While some of the extra processing could have been handled by the extra CPU, if Quartz Extreme is included in the preview (which I believe it is), it appears that both hardware accelerated screen draw (Quartz Extreme), and optimized Quartz code in general, are contributing to performance on ALL cards. In short, Quartz "should" run better that it does currently (where all the processing is being done by the CPU), simply because the code is cleaner, and work is being done by the previously dormant graphics card.