There will be no performance increase for AE with a graphics card upgrade unless you're using a plugin that takes advantage of it. (There aren't many that do.)
Definitely max out your RAM. AE will want as much of that as you can give it and a faster processor wouldn't hurt.
It's hard to give any sort of predictions for speed increases since there are so many variables at play. This thread [link] on the Adobe forums should give you an idea of what settings you should do for your RAM usage and whether you should use multiprocessing.
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