Both would do,but if the "gold rated is the same price or just a little more expensive would that be my choice. The 650watt is prettty much overkill and really not the same quality as the 550watt gold i bet.
If you have a budget and if the next is where you can buy could we help with a choice,give some more options if you want.
http://pcpartpicker.com/
country can be changed at the righthand top.