UPDATE/EDIT: If it matters, I may add on a third and/or fourth GPU to the system later. Running a 1k watt PSU in the rig right now with ample space and airflow.
Do you plan to use water cooling ? How else can you fit 4 cards in one MB ? 
I would recommend mining on Windows, of course. Unless you are going to install more than 2x5970 or 4x5870
GPU limitation on windows is no longer an issue with 11.6 drivers