It would indeed run that game at 1920x1080 at high settings. Though you might want to look at getting a 7850 to make sure the game will run as smooth as you'd like it to as the 7770 might have some slight stuttering in that game at a higher resolution.
If you're playing at 1680x1050 or lower though the 7770 is more than enough to max out that game.
its newer technology, so it consumes less power from the PSU, it simply performs better(it performs just under a 6850/gtx 460 level). It will run cooler and overclock higher(probably) than the 550ti can.
but what actually makes HD 7770 more powerful than GTX 550?? and also what "GTX" GPU can perform the same as HD 7770?
Different tests and reviews show that the 7770 is faster. The cards use different architecture so a direct comparison isn't necessarily accurate to explain 'what exactly' is the reason. If you want a GTX card you'd be looking at a 560.