There is no such thing. Memory allocation is dynamic and if it isn't using more, then it doesn't need more. That aside, just using more memory wouldn't do anything in terms of accelerating renders and such. As a 32bit app, AE by itself can never use more than 4GB, anyway. To exploit the benefits of multicore machines with more RAM, you will have to set it up so as to use Multiprocessing. On a system such as yours, it might be possible to use 2 additional background instances then, assuming you have a quad core.
Mylenium is right. Here's an FAQ list entry about this:
But, really, the question "How can I make After Effects use more RAM?" isn't as important as "How can I get more done faster?". For that, see "Improve performance".