Nobody can answer a question like that.
It's got way too many variables, like your other hardware, what games you want to play etc.
It's safe to say there will be a nice bump, but it's not going to be a vast leap in most games. The Q9550 was a great processor and while it's getting fairly old, it's still quite solid. When you consider that a lot of games are much more dependant on your graphics than your CPU, you can see how difficult it is to make any real kind of assessment. On the other hand, games like Skyrim and the recently benchmarked Far Cry 3 are pretty CPU taxing so a newer generation CPU will make a much bigger difference. http://www.tomshardware.co.uk/far-cry-3-performance-ben...
What is also significant is that the 8350 has 8 threads. While for gaming this is essentially pointless, as an overall picture it has a lot of potential benefits when you consider the open browser tabs, media players and the like people often leave running. In that kind of scenario, the 8350 will have a much bigger advantage.