The 7850 is pretty much the absoulute best value in a GPU, but if you don't have the money, or your CPU or PSU is holding you back the 7770 will be much better than the 550 Ti.
7770 newer, 28nm not 40nm so more power effecient, slighty faster but really you'd be better off spending the extra to get a 7850 as you get a lot more power for a little extra money.
Alternately if money is a factor you could try getting a used 5850 or 6850 of eBay.
I agree with them the 7850 isnt that much more expensive. 550 ti is a pretty weak card. Your better off just buying the new 650 ti. How much are you wanting to spend?
The 7850 is pretty much the absoulute best value in a GPU, but if you don't have the money, or your CPU or PSU is holding you back the 7770 will be much better than the 550 Ti.