It is my belief that AH is nearly all about the CPU. The GPU is important also, but of course GPUs come and go. It was not too long ago that the Geforce 200 series was so capable, but now they are not even capable enough for PhysX on the latest GPU types. Not that PhysX is even needed for AH, but if you have a 600 series, or 700 series GPU then the 2## GPU for PhysX will only slow you down. For a CPU I think a 3.0GHz CPU is bare minimum, with 3.6GHz (or higher) being the ideal. I use a 4770K at 3.9GHz.
For memory it depends on your OS. For AH you should use Windows 7 64-bit. Do not use Windows 8. W8 has some fanbois around, but the OS is hopeless and not worthy of the upgrade price. 8GB is all the memory you need for W7 64-bit, but you can go to 16GB depending on what else you use your system for. While it is possible to go even higher, it will not be worth the price you have to pay. Regrettably, there are few motherboards that remain stable with four DIMMs in use anyway and you can really pull your hair out trying to get it right.
For the GPU I would go with the 600 series myself. You can still find the 680s around, but they will be expensive. Worse, the best units are the ones with the most VRAM onboard, which pushes the price even higher. But wait! Since we also suggested waiting you might be seeing a new GPU hit the market at any time. You might consider looking at the reviews of the 750Ti Maxwell cards. There is a Wikipedia article on the new GeForce 800 series, but I wouldn't swear to the validity of everything they have there.
http://en.wikipedia.org/wiki/GeForce_800_SeriesI'm waiting on Volta myself.
I would put off buying a PSU until you know what the entire system is likely to be. I could write a book on why that is.
I would also plan on buying two hard drives at the least. I run Aces High on a second drive, because when I tested it that way it runs smoother.
Choose a MB that offers SATA III 6GB, and USB 3. USB 3 is even faster than Firewire 800. Read the QVL for the MB you chose and pick out RAM that runs well without overclocking.