Advantage? Zero. That's simply not how most stuff in AE works. Perhaps this years edition with the new threading model and other performance optimizations will make it worthwile, but until then beyond the simple "the more RAM, the better" is the only thing and even that mustn't necessarily be true...
I wouldn't say the advantage is zero. I have noticed a difference on certain projects I've worked on with having more cores. I will grant that on some projects (with the current version of AE), it wouldn't help at all. Some things in AE simply just run on one core.
Regardless, the clock speed of the processor(s) makes a huge difference no matter what! So pay attention to that.
If you're really interested in performance in AE, watch this: http://www.adobe.com/cfusion/event/index.cfm?event=set_registered&id=2490687&loc=en_us It's a (recorded) live event where members of the Adobe team discuss how AE and Premiere make use of system resources. Again, in the next year or two, as the AE team works to rewrite how After Effects works, all of this could change.