The PSU should be more than enough.
The 560 is faster than the 280 and much more effiicient, so if that's in your budget, I suggest going with that. The 280 is much older and uses more energy to do less. Unless you can get it and need to get it considerably cheaper, the 280 is not a great choice.
I doubt you will have a bottleneck, but should that occur, you can just lower the options that are CPU dependent.