For After Effects, the factors that matter most are, in order, these:
1. RAM and CPU
2. fast disks (preferably SSD for caches)
3. GPU
At the risk of being redundant, I really need to stress that the GPU is _last_ for After Effects.
For Premiere Pro, the GPU is somewhat more important. A system more for Premiere Pro would have less RAM, with the money saved spent on a better GPU.
Don't skimp on the speedy disk drive(s).
See this page for information about hardware for Premiere Pro and After Effects:
http://adobe.ly/pRYOuk
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Todd Kopriva, Adobe Systems Incorporated
After Effects quality engineering
After Effects team blog
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