1. No harm in trying both out and see which one you like better.
2. The Zalman 9500 is likely to underperform compared to the Hyper 212+. But, note that you won't be able to mount all four of your RAM sticks in the normal CPU cooler fan position since the tall heatsinks have clearance issues with many tower CPU coolers. If I was in your position, I'd just pick up the Hyper 212+ and put the CPU cooler fan in pull position.
3. 650W would be the minimum amount I'd suggest that fits a single GTX 580 with a little room for CPU overclocking. For SLI, 850W+.