2. Not to nitpick above, but wattage is important. In fact, they are exactly deliverable load (joules of energy/second). It is true that it's not the only important thing in a PSU, so you want to stay with a reputable vendor. Corsair is decent. I would not build a new machine these days with less that 800, assuming a current or future plan for SLI.
4. I'm not a big fan of overclocking in general. I used to be a diehard OCer, and it's just not worth it. Nothing pisses me off faster than a computer that isn't stable, and while I'm not suggesting all OC's aren't, it's a lot more probable when you're pushing limits. Plus, if something does go wrong, it's always the first thing in your mind.
5. You can 5box with it. I was using a gtx260 for a time, got about 40fps on an i7 with 12G and all the slaves capped at 15fps. Highly recommend EVGA.
6. Not a fan of the SSD for this application, tbh. I've tried two different ones now and noticed very little different. Having tons of memory, I think, somewhat invalidates their usefulness for boxing because we are always using 5 copies of the same data, which will be cached in RAM.

Invest in a good monitor. Don't listen to the "it's too big to be useful" crowd, there is no such thing.