I have the exact same laptop
OC'd to 620/1050 I get to 73C, I'm thinking there's dust somewhere on your laptop, because in spite what alot of people say, this Dell is very well cooled, so if its not cooling there's something wrong. At stock speeds, I rarely go over 68C. Limits are different on every single card of the same model, I've seen 5650m's only overclock to 600/900 - I say only, but its still quite a good increase in performance - but generally 5650's are cool running cards.
Whenever I go to use my laptoip for ANYTHING I prop it up on 5 dvd cases either side on a flat surface, if you don't have that sort of time or environment, I would make one.