The absolute best hardware you can buy will probably be on the marker the day after you place your order. I'd just go through the system requirements for the program you plan to use most often. If you are mainly an effects house and your primary job will be to create shots or short sequences that are full of visual effects and motion graphics then look at AE's system requirements. If you want to produce finished films that are longer than a few seconds then you want to look at the system requirements for Premiere Pro and compare them with the system requirements for AE. In it's current state After Effects calculates every pixel one frame at a time by decoding the original image, adding in the effects layers and everything added to the image, makes any calculations necessary from the previous or next frames, then compiles an uncompressed frame that is sent to be compressed and written to disk. It is **** this one frame at a time utilizing as much memory as that process takes to complete (that's why you seldom see 100% of memory and CPU power used) then moves on to the next frame. This is always going to be slow until the GPU can be added into the mix. My point is that you should make your buying decision on the system requirements, your budget, the customer service offered by the company building the system, and then go from there. Get the most powerful system you can afford with the fastest processor and storage that falls under the approved and tested hardware that Adobe has on their list, because if it is on their list then it has been tested. If you want to experiment with untested hardware then make your best guess at what's going to work and expect to run into more problems optimizing your machine than you would find in a tested machine.
The last step in the process is to refine your workflow so you don't have creative folks sitting around waiting for rendering. Even with equipment that is a few years old I very seldom sit around waiting for renders. AE and Premiere Pro are efficient enough now that you don't have to do that if you work out the right workflow.
your dual xeon build may end up costing between 5-8k+, and only perform as well as a $2-3k i7 build. the dual xeon's won't outperform an i7-5960X until you get the E5-2630 v3 or higher, and you will also be looking at 64gb+ of ram to feed all the cpu threads in AE. do not go over 32 cpu cores/64 threads as adobe's software currently isn't programmed to use more. if you don't need workstation grade hardware/service, you could buy an i7 computer for $2-3k. if you still want to burn $8k+, you could also buy another one or two fast i7 towers dedicated to rendering premiere and AE projects, and/or build an AE render farm.
If you want a custom build designed specifically for your workflow and editing requirements go to one of our very experienced two forum members who specialize in video editing like Eric at ADK or Jeff at Safe Harbor. These people live and breath video editing and stand behind their products with your editing software. I my opinion it is a major mistake to buy the name brand computers for video editing.
If you want to build your own computer go to our Premiere Pro BenchMarking (PPBM) website and start reading.