PSU calculators always overestimate your power requirements. There's nothing wrong with a bit of overprovisioning but a non-OC Haswell CPU + GTX 770 + one HDD won't consume more than 300W under load so 750W really is a bit excessive. The normal 840 uses TLC nand which is especially bad for lower capacity (120GB) drives because it will degrade quicker. Considering the M500 is cheaper there's no reason at all to go for the non-Pro 840. To be honest I think you should go for the M500 even over the 840 Pro considering the price difference and I doubt you'd be able to notice the performance difference.