dedicated PSU for what?? water cooling. Its not needed. But for your setup you would need at least 1k power supply. I would recommend 1.2 k
For your setup i would recommend dual loop setup. One for CPU, and mobo if you would consider watercooling it as well and second for grfx cards.
So ie. 2 x 320 mm rad, 2 pumps ( ie. swiftech mcp 355) , grfx blocks, CPU blocks, NB,SB and mosfet blocks( if considered) and hose, coolant. barbs or compression fittings.
For details see here:
Basic Water Cooling Guide for Beginners