Companies recommend significantly higher than required amounts for watts because they have no clue what you could be running on your PC, and they account for the ENTIRE system not just the GPU. They also don't know if your PSU is peak or continuous.
500w continuous (Yours) > 750w peak
The reason you have more than one CPU cable is because some motherboards require more/less CPU pins.
My thoughts on the 960 is that it is a SLIGHTLY better GPU that doesn't justify the $100 price difference. They are GPU's of the same tier, and at the price that 960's are going, I'd rather spend a few extra bucks and blow it out of the water by catching a 290 on sale.
McDohl