this is the power consumption with an over clocked sandy bridge-E ( that will use 48 watts more at stock)
all the 660ti are overclocked also and except for one they are under 400 total watts. the max load you want to put on a PSU is 80% of it continuous
load for best efficiency. so 400/.8= 500 watts
also lower quality PSUs have weaker 12 volt rails, where you really need to draw power from for the CPU and GPU. a better quality will have 80% of its power on the 12 volt rail or 400 of 500 watts. so a minimum of 34 amps of 12 volts would be necessary; though just the CPU and GPU won't use more than ~300 watts.