A 7750 will work with as little as a 300w psu. It is powered only by the 75w pcie-x16 slot.
Gigabyte is a reputable company, but I know nothing about the quality of their units.
As to quality, this guide may help you. Do not go cheap on the psu, it may turn out to be very expensive if it gails and damages everything it is connected to: http://www.eggxpert.com/forums/thread/323050.aspx
Do not pay much attention to gold/silver or even 80+ efficiency ratings. You will not recoup any extra expense paid for more efficient psu's.