How much power supply do i need

Depending on the 12 volt rails power, you only "Need" about 400(and even that is over estimated), but 500-650 would be a good place to start(for long life and good efficiency).

Quality names like

Antec,Corsair,Seasonic,XFX are good. There are many other good companies. If they are giving 80-90 % of the total wattage to the 12 volt rails, it's a good sign.

Amps * 12(volts) = watts and you must find a total wattage or amperage for all 12 volt rails.

I am using this 300 unit as a idea of what to look for

288 watts(this is the combined 12 volt rails as you see, adding up the rails does not work) on 12 volt. good. And the majority of the power is on the 12 volt rail, also good. I have seen 450 and 500 watt power supplies with FAR less 12 volt power and that is what your system uses the most of.

Try this link by Antec http://www.extreme.outervision.com/powercalc.jsp

I'd go for at least a quality 550-600 watt. You'll be drawing maybe 300-350 by my estimation (not sure how much power watercooling gear uses).