Did you find any benchmarks regarding scaling? I looked at the same two CPUs as well but they are so darn expensive..if the prospect of prices coming down quite a bit at some point a second 2643 later on could be much cheaper than a single 2687 upfront..
Here's the CPU Skinny on Maxwell Render [
http://support.nextlimit.com/display/knfaq/System+Specifications+FAQ ]:
"1. Which is the best hardware configuration for using Maxwell? ...
Each hardware component is in fact crucial in a particular part of the 3D process:
The CPU is the most important component for fast renders. Get the fastest processors you can afford, plus with network rendering,
Maxwells render speed scales almost linearly when adding more computers to contribute to the render process. At least a quad-core CPU is recommended.
RAM is where the scene information is stored during the render process. If your scene has complex geometry, huge textures, set to render at high resolution, or has the MultiLight feature enabled (which increases the amount of RAM needed to store all the emitters information separately) then you may need a computer with a lot of RAM (4GB and more). A usual configuration for a rendering / video compositing computer is 12-16 GB of RAM.
The graphics card is not involved in the rendering process. It is only involved in the openGL camera navigation when you are creating your scenes. A gaming card will be sufficient for most tasks, unless you plan to work with scenes containing many thousands of objects and/or require antialiased viewports which are much better handled by professional graphics cards. Their added advantage is that their drivers have been certified to work properly with different CAD / 3D applications such as SolidWorks, Rhino, Maya, 3dMax etc. It is also recommended to have a card with large on-board memory (1GB or more) if you plan to work with many large resolution textures and want to view them in the openGL viewports."
So a dual processor build, using a $517 (US) Supermicro DAX [
http://www.superbiiz.com/detail.php?name=MB-X9DAXF ], would not be overkill and since it does not use the GPU for rendering, the limited no. of x16 slots would not be much of a loss, plus you can overclock it about 1.075X. But how important is that in the scheme of your other computer uses?
Consider getting two of these: Xeon E5-2643 v2 (Ivy Bridge), 6 cores / 12 threads each, running at 3.5 GHz base w/o overclocking and that turbo boosts (TB) to 3.8 GHz w/o overclocking ( having 25 MB cache and a TDP of 130W) [
http://www.cpu-world.com/CPUs/Xeon/Intel-Xeon E5-2643 v2.html ]. With the DAX motherboard you may able to get both of them to overclock by 1.075x, giving you 12 processors/24 threads running at 3.76 GHz at base (which PS and Ill. would love also) and that turbo boost to 4.085 GHz. The V2s retail each for about $1,552 (US). If the budget is lower, you could opt for V1s - they're $855 (US) each, quads (not hexes) and have a base of 3.3 GHz and a TB of 3.5 GHz, which would be about 3.55 base and TB of 3.76 when DAX aided.