The watt ratings is a measure of the maximum power it is certified to sustain. In a way you can think of it as is saying you can pull x amount of electrons through the wall into your pc without damaging the PSU (overheating, bursting into flames, etc.) A 650w model can support a GTX 580 so there is absolutely no reason to go above that. As has been said you could even drop to a 550-600w model and still be safe. The reason behind not getting an overkill PSU is to save money (rather, not waste it). A common misconception is that a 1000w PSU will always draw 1000W from the wall, not true, only what the computer needs it to provide and a little bit to account for the fact that they are not 100% efficient (i.e. computer draws 550w but you will measure 590w from the wall)
That corsair model is a very good one and will be plenty for you.
Edit: I may have glazed over someone else mentioning it, but Sandy Bridge uses a dual channel RAM architecture, you have a triple channel kit. I would suggest either this 4GB kit
, or this 8GB kit
, 8GB is preferred due to the low current price of DDR3.Edited by dinkledork - 10/10/11 at 1:22pm