Comments 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.
Interesting article
I am definitely looking forward to reading more 
I have built a workhorse machine Dell T7400 with – 20gbs ram – 8core xeon processors – nvidia quadro fx 4800 video 30″ 2650×1600 monitor, and 64bit Vista software. The machine moves good until I either use Premiere pro and or After Effects. Rendering is slower than my laptop dual core. What is the magic configuration? I surely don’t have it.Max[response from TMGK:It's best to post questions like this to the Adobe After Effects user-to-user forum:http://forums.adobe.com/community/aftereffects_general_discussionSeveral motion graphics and visual effects professionals help people on that forum to deal with issues like yours. I monitor that forum, too, so I can help you to get an answer there.When asking such a question, please supply information about the version numbers of your software.]
Hi. I got a doubt about this. When you say “per process”, do you mean per core?. If so a quad core should need 16 GB of ram, even if it works with HT. But if i am using a Hyperthreading capable quad or i7 that “creates a double process” per core , should that mean that i would be needing 32 GB of ram?. I am confused about this and i do appreciate your advise.[response from TMGK:After Effects only uses the physical processor cores (not the virtual processor cores created by hyperthreading) for Render Multiple Frames Simultaneously multiprocessing. So, a quad-core is just a quad-core in this context.Each processor core can run one After Effects background rendering process.]
AECS4 *does* see virtual HT cores on OSX 10.6. With HT on AE sees 16 cores on my MacPro, with it off it only sees the true 8 cores (this is visible in the multiprocessing settings, google and you will see discussion about it). Apparently AE *shouldn’t* be seeing the virtual cores, but in some cases it does.I seem to get better performance in AE by turning HT off on my system.[response from TMGK:There's a difference between "see" and "use". The Memory & Multiprocessing dialog box can see them. But Render Multiple Frames Simultaneously multiprocessing can't use them.]