System configuration for after effects

You have to start with a system that meets the After Effects System Requirements and just start adding from there. Stick with the recommended GPU series, add even amounts of GB of ram so you can reserve at least 4 GB for other apps. Pick good hard drives or SSD's for storage and make sure that the interface is fast if they are external. Never scrimp on cables. Unless you like fiddling with drivers and configurations all the time don't build your own machine, but buy one with a good warranty and service contract. 

When you have all that together remember this key fact. After Effects is NOT a video editing app. It is designed to create shots and short sequences that cannot be created using an NLE like Premiere Pro. If a shot does not require visual effects, motion graphics, or animation then it does not belong in an After Effects project. AE is incredibly capable and complex so count on spending a lot of time with the User Guide to get to understand the workflow. 

No matter what system you put together or what OS you choose, if it is among the best systems available for After Effects today it may be at the bottom of the list after a couple of new versions have been released.

I hope this helps. It would help us help you a lot more if you told us what your goals are and if you can give us an idea of your budget. You can get a lot of work done and turn a fairly good profit if you know what you are doing and spend around $2500 machine. You can easily go broke if you spend $20,000 on a machine and don't learn how to work efficiently and meet deadlines. If you can just barely afford the minimum machine, and you are really clever, you can still do amazing things, it will just take longer for them to render.