1. The better MB would obviously allow me to expand to other cards, but if I add other cards, they probably won't be 5970s due to availability and cost, so it'll be a mix-match setup. Other plus on the better MB would be that I could potentially expand the computer to other uses in the future.
2. Cooling: Recommendations? Keep in mind it's going to be living at an office, so emphasis needs to be on quiet.
3. PSU: Looks like the 5970s pull around 300W, so I'm wondering if the 750W PSU will be sufficient if I want to expand to 2 cards in the future.
1. Up to you.
2. 5970 is the antithesis of quiet. It is brutally loud. It also generates immense heat. 2 going at full bore will easily increase the temp in your office by 10-20 degrees. Only option is watercooling or accelero 5970 (if you can find one).
3. One, yes. Two 5970s, not reliably.