Considering the G550 is based on Sandy Bridge you should be seeing a pretty substantial increase even at the same clock speeds. It's also built on a smaller process so power consumption and heat output should be fairly lower as well.
Gaming is very GPU dependent, so in the end the focus should be mainly on that. But yes you still want an overall powerful system and the G550 would be a better pick in that regard. It's hard to pin point exactly how much of a performance gain you'll see, as games differ in what resources they need to perform best, though i am sure there are reviews of both of those CPU's out there showing in game performance numbers.
But overall the G550 is the better pick in regards to performance.