If you are looking to upgrade your power supply and aren't sure what the minimum is or what options you might have, EVGA has a questionnaire to help you out (ostensibly to sell you something, of course)
http://www.evga.com/power-meter/
after you complete the EVGA questionnaire, you'll get a discount coupon to use if you buy a power supply through EVGA's website.
Here's something to keep in mind - After I purchased my GTX970 SSC and registered it, I got a 50% off coupon from EVGA to use when buying an EVGA power supply AND an EVGA mouse. I got the 850 G2 power supply and their X10 Torq mouse for $119 total cost, free shipping, to me after discount.
here's their current B-stock for 900-series cards