most X58 chips use 130w@ stock, 250w doesnt give you much head room especially when you consider the amount of volts required.
take the 580 for example, when the card is stock it uses about 250w of power under load at a voltage of 1.1. when you increase the voltage to say 1.5v it can use as much as 500watts of power as explained in this thread----- i cant find the thread.......
i think tin did it and its on kingpin cooling.
anyway what im getting at is an overclocked 960 will chew thru a 250w phase in no time, and -20c wont get you very far, you can do better than that with a AC water chiller like nate and Q56 used to use