but 5770 is just 128 bit card!
although the 5770 is 128bit, its runs its memory clock faster but that is besides the point. The 4xxx and 5xxx are different series of cards so you cant compare their statistics against one another. Your best tool would be review benchmarks to see comparisons.
The 4890 is faster then the 5770 but not by much. The 5770 is cheaper, offers DX11 (if you have Win 7) and runs cooler. Both are fine cards and you cant go wrong with either one.http://www.tomshardware.com/reviews/radeon-hd-5770,2446-7.html