This is the power Supply I am using. http://www.newegg.com/Product/Product.aspx?Item=N82E16817371044
Note: The Pc I am using was built for me by a company called eCollegepc. I had them put in all of the parts that I asked them to, since I have no experience building yet. I'm reading some of the reviews on that power supply, and it said people have problems with burning. Does that have something to do with over using, or overdoing what the power supply is capable of doing? I've had this computer since April to today and i've had few to little problems that weren't easily solved.
"EDIT: why do you want to add another GTX 660?"
I don't know, wouldn't it run better or would it not make much of a difference?