If you're doing anything outside of After Effects, I'd probably recommend quad core processor. After Effects isn't well threaded, so when doing multiprocessing, it requires multiple copies of After Effects to be run concurrently; each using large amounts of ram.

However, other programs make great use of multiple cores without the huge RAM allocation. Mental Ray causes all 4 cores to crank up to near 100% utilization when rendering. Softimage XSI's ICE particle system is very multithreaded, allowing near realtime control over LOTS particles.

I'll never buy anything less than quad core now (still waiting for quadcore laptops to debut, although Dell has one ).