I'm thinking of upgrade to a GTX 660 and my current card is a Radeon HD 5770. They both require the same amount of watts which is 450w and I have a 460w. The Radeon and the Power supply both came with the computer. Here is my computer
450w is more than enough, it max power drawn from the card is 140w, the recommendation is a 450w psu.
The 5770 consumes a bit less than a 660 gtx, but it might do for that psu, it's not a huge difference that will make the psu shutdown or burn.