First of all CUDA does nothing in AE, and the graphics card should certainly be at the bottom of your list for required components.
I have 48GB in my workstation, and AE will easilly use all of it. I have 8 cores with hyperthreading turned on, so 16 potential threads.
I normally run 8 AE threads, with 3GB/thread, and 8GB for other apps.
For renders than need lots of disk access, I might reduce the number of threads.
Remember that even if each extra thread can have max 3GB (not sure why), the main AE application will eat into your memory very fast if you work with complicated HD projects. Especially in 32bit.
I'd certainly recommend you having 32GB in an 8 core machine. Whether having 48GB is worth the extra cost is hard to tell.
On a 12 core machine though, I think you should seriously consider 48GB if you can afford it. Certainly no less than 24GB, or you might as well go with an 8 core.
But as Roland also mentioned fast disks are important too.
In the end this will all somewhat depend on what kind of projects you do.
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http://www.jonbagge.net
Jon Bagge - Editor - London, UK
Avid - FCP - After Effects