Look at the specs on the power supply. What is the combined 12 volts power rating?
I have a 5770 in my i5 750 SFF system running on a 300 watt power supply. Lucky to see the system get much past 150watts from the wall unless I compress video and play games at the same time. Then again the i5 750 is not a power hog and undervolts well, plus a miniitx board is very power efficient.