Watts on the psu are not the question/answer. Look at the side of the psu and see how many amps are available on the 12v rail(s). I would guess it is a dual rail unit with one 16a and one 18a rail. A 660 ti is a 225w tdp or there abouts and that means it will need close to 30a. Even if you use a 6 pin from each rail, you going to be short of power at the end of the day. Also, Dell OEM psu's are not known for their quality - lowest bidder like the government.
If it was me, I'd look for a quality psu from corsair (HX or TX), seasonic, xfx or antec in the 600w range with 45a or more on the rail(s).
Mark