RAM is important, but since AE is a 32-bit application running on a 32-bit OS (for the time being) it can’t possibly address more than 4GB of RAM at once (and less in real world terms, Windows only allows 2GB and Mac OS 10.4 3GB). So there’s a definite ceiling in terms of RAM. However, if you’re running After Effects and Photoshop at once (as I often do) you’ll want extra head room in terms of RAM.
Processor is by far the most important aspect, but it’s important to balance this with RAM.
One note: although multiple cores/processors will help if you have GridIron Nucleo, After Effects is itself not able to tap into the benefits of multiple cores.