the hd7750 is the best choice. its a card tailor made for people wanting to upgrade pre built HP and Dells.
7750 while being more powerful than 6670 also consumes lesser power which is amazing. and the card is great in performance.
you will easily be able to play crysis 2 and bf3 at med/high at upto 1600*900 res.
i also have 7750 and benchmarked its performance at 1440*900.
at gamer settings which is the lowest quality setting i got a solid 60 fps and at hardcore setting which is the highest setting you still get 35-40 fps. pretty amazing little card that does not suck.power.
go for it !!!