Hey jtk77.........
I posted an answer to this back on 05/15/02 called:
"How To: Optimize your RAM Playback"
It explains a lot of what you need to know:
Here it is again:
How to Optimize your system to get Real-Time RAM Playback:
Many people have asked about how to optimize their systems to get full RAM playback. They seem to have problems, despite fast CPU speeds and high amounts of RAM.
The reality is that real-time RAM Playback can depend on both your computer and graphics card: Some computers are not able to play back from RAM at full frame rate in the viewport because the graphics card simply does NOT have enough 2D performance. In other words, a better graphics cards will deliver faster playout.
As for the playout: It has nothing to do with the number of video or graphic layers, because once something is rendered to RAM, the images that are stored are independent of the processing that was required to create those images. A good idea for improved RAM playout is to optimize your Rendering Settings: set your output to the "Color" option and NOT the "Color+Alpha". This is because Color creates a 24-bit images in RAM, whereas Color+Alpha creates 32-bit images (RGBA.) Obviously, 32 bit will play slower in the viewport due to the extra data! Note that "Color+Alpha" is the default option.
Also, if you are using a frame buffer for live support, you can also size-DOWN your viewport and rely on your video monitor for playback. Or, conversely, switch OFF your frame buffer to speed up your RAM playout on the Viewport.
This tip applys to MAC/PC versions, and Combustion V1 as well as V2.