Upgrading from a HD7570 to a HD7770 should net you a noticable performance boost, but it really depends on what kind of results you are expecting, and how hard you are pushing it. The HD7770 is a solid entry level gaming card right now, but most games will eat it alive if you try and crank settings up. As you didn't mention any other details, it's also possible another part of your system (CPU would be the likely culprit) is holding performance back.
A general piece of advice would be to make sure your drivers are up to date, it's a cliche but it can make a world of difference.
For more specific info, you'd need to provide more details, specifically-
Your entire system specs
Which games you are playing
What settings you are trying to run these games on
What sort of fps you are getting
Without those details, everything else is basically speculation.