Karl Soule just posted a link on his blog to a new white paper about how you can get a big performance bump by running CS4 Production Premium (or just After Effects CS4) on a 64-bit operating system, especially if you cram a lot of RAM into the computer.
The trick to making maximum use of the RAM in your computer with After Effects is to set the Memory & Multiprocessing preferences, including Render Multiple Frames Simultaneously.
With Render Multiple Frames Simultaneously, After Effects can start separate background processes of the After Effects application to render multiple frames at the same time. This can really speed up renders, both for final output and for RAM previews.
2GB per process is the limit on 32-bit Windows without special (and some say somewhat risky) configuration. And, of course, the OS can only see 4GB, so that’s severely limiting.
Nearly 4GB per process is the limit on 64-bit Windows, with no special configuration required. ~3.5 GB per process is the limit on Mac OS (though the foreground application gets a little less because of the Mac OS UI libraries). Because these numbers are per-process, and because a 64-bit operating system can see a lot of RAM, you can make use of around 30GB of RAM in an 8-core computer.
More performance tips for After Effects CS4 can be found in the “Improve performance” section of After Effects CS4 Help on the Web.