Okay, here's the deal.
AE does support GPU accellerated previewing and to a lesser extent some plugins are GPU accellerated.
But as projects get more complex, the GPU maxes out fast - especially when working at HD resolutions.
My advice would be similar to Darby's. Get a fast Core2Duo CPU, and 4GB RAM.
Bar3nd
Raamw3rk - digital storytelling and visual effects