The 7750 should perform about as well as the 6700's but with dx 11.1. I suggest you get 7750 as its more up to date and you will have no problems finding 7750's later for crossfire. AMD stopped production of the 6000's so it could be hard to find a second later. Also your 400w should have an easier time pushing this card as it requires a bit lower power.http://www.newegg.com/Product/Product.aspx?Item=N82E16814161403
notice that the7750 beats the 5770 in directx 11.
It draws less power than even the 5750.