After effects is fun, I love making videos.
Anyway, the I5 processor will do just fine. Since you have to move allot of stuff typically on and off the hard drive I would recommend a solid state disk and allot of ram before worrying about the GPU.
A GPU I would recommend, if money is no option, is the Quadro 6000. The tesla GPU's have the best performance of any GPU in Adobe software. http://www.nvidia.com/object/after-effects-cs6.html
But I don't think you want to spend 4k on a graphics card, so get the next best thing.
Get a GTX 470 about $245, then follow the instruction in the article below to flash it to qaudro 6000 specs. http://www.techarp.com/showarticle.aspx?artno=539
If you do this you will have a professional quality machine for 1/10'th the price.
Hope that helps, have fun!