With 6 7950 cards, you should go with 2 rigs, both wit 3x7950.
For each rig you should get decent PSU, somewhere in 800W range (not less than 750W) with single 12V rail design.

Price difference between 2x 750W PSU and 1x1500W PSU (for 6x7950) is more than enough to buy 1 motherboard, CPU, HDD and 4GB of RAM.

You will avoid any potential windows problems when running 6 GPUs on one system (I didn't see a singe system with more than 5 GPUs), and you will have couple of inches between cards, unlike with 6 cards where you can't get more than 1". Like that you will have better cooling, and quieter system.