AE renders on the CPU, not the GPU. AE likes fast multicore systems with lots of RAM.
There are a few third-party effects which are accelerated by the graphics card, but for general work, spending the money on RAM is usually the better bet.
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I agree with Walter - RAM and cores is what AE likes. Unless you plan to use new generation 3d plugins which make use of the GPU, any decent card will do. For using FreeForm Pro or ShapeShifter AE you can check the recommended cards on mettle.com