As far as a actual percentage I can not help you. What you should do is look around for benchmarks of the apps that you use your machine for. You should use the AMD 955/965 as your reference chip. Since your current clock lands right in the middle of the clocks of the 955 and 965.
I do not think its worth it to switch. Well let me rephrase that. I know it is not worth the money to switch. For the switch you would need to get the cpu of course. Plus a new board/DDR3 ram. No way in the world would I do it.