i believe the color depth was only relevant when the video memory was smaller than would a big application produces when you have many windows open.
This is true. But it was only relevant in the "old world era" where most of of the Macs came with only 256k, 512k, 1Mb or 2Mb of video RAM. With 16,32,64 or 128 MB of video RAM in most of the nVidia and ATI cards this problem it is only a gamers concern
@mactron: and how does the load on the radeon influence protools?
In a graphic environment like Mac Os 9, there is always some amount of CPU/GPU time dedicated to graphic tasks (QuickDraw). I don't know the exact amount of time that Pro Tools uses on to this. It is hard to measure. And It is different in each video card and Mac model.
But if it is 30% in a G4@500 it will be 10% in a G4@1500, and if we double the "Video Power" ( installing a faster video card or reducing the bit depth ) this will be a 15% in a G4@500 and a 5% in a G4@1500. This leaves more CPU power to Pro Tools, the Mac Operating System or any other application...
I know that Pro Tools can be run in a very complex hardware setup, with a considerable amount of DSPs, but anyway it always need to draw things on the screen