There is no specific forum but there are some who posted the specs of the machine they are building.
here are some:
Best/Fastest hardware for solidworks? That's a bit questionable because it will depend on what specific work you're doing. My advice is that try to contact your VAR so that they can advice you the configuration of your machine base on what work you will be doing. Or if you're planning to build one then try to post here the specification of the machine you want to build and also the work that you will be doing so that others can give advice and experience for the hardware and its performance.
Workstation hardware cost $$$$$$$ and it does not mean that hardware with the highest price is the best (but most of all) other hardware are pricey because it has so many feature. By knowing the work you will be doing you can build a machine with a blazing speed and a great satisfaction without spending $$$$ to features you do not need and building a machine that is overkill.
Hope this Help,
(In the world of CAD/CAM, even you have the greatest machine with blazing speed and overkill, but you do not have the skill then even an average machine can beat the greatest machine into hand of a skilled user)