...Also I am wondering if I should upgrade the PSU to a higher wattage to reduce its load.
I have seen a review of the Asus X79-Deluxe with the R9 280X and the 4960X CPU. The maximum power draw under load was measured at 475 watts (from the wall). I guess you would might see a bit less than this but potentially it would mean stressing a 660w psu to something like 60 or 70% of its capacity. It could cope but there would certainly be an issue of PSU fan noise at these stress levels. So upgrading to something like the Seasonic 850w would make sense if keeping PSU fan noise down is an issue.
So I am now considering some other options for the case and I keep changing my mind on which would be best. Cases I am considering are.
Corsair 330r carbide
Fractal Design Define R4
Antec solo II
One feature the Antec p280 has is that two intake fans can be mounted on the internal face of the drive cage, which places them directly opposite the PSU and graphics card. Putting fans here would probably be more effective in keeping down graphic card fan speeds and noise than an aftermarket cooler. The p280 is supplied with three fans, but has five fan positions. Given a power setup like this and that the p280 uses 120mm fans it would make sense to fill all the positions.