Regarding the GPU, you don't need a high-powered 3D-capable card for AE use. The OpenGL features that AE uses should be easily satisfied by much less than a top of the line gaming or 3D workstation card.

Here's a list of currently tested cards for AE CS4:

<http://www.adobe.com/products/aftereffects/opengl.html>

I've never built, or purchased anything with AMD procs, but the Intel ones I've had have always worked out well. What OS were you thinking of running? If you're sticking with windows XP 32, max it out with 4 GB of RAM. If you're getting a 64 bit OS, get at least 2 GB of RAM/core (or processor). In reality, get as much as you can afford. AE will eat up all the RAM that can be thrown at it, which is somewhat OS dependent, and also dependent on the nature of the app itself. Currently, AE is only a 32-bit app, but you can use multiprocessing features built-in to the software to launch background instances (which will eat up some more RAM), or, you can invest in GridIron's Nucleo Pro, which will also launch background processes, with a few more bells and whistles (some very useful ones at that). A 650w PS should be more than enough. Get some fast HD's too. Ideally, you'd have one for the OS and your applications, another for your source footage, and a third for "render" drive.