What's the purpose of your PC?
For CPU intensive tasks a Core i5 will be around 20% more powerful than C2Q clocked at the same speed. You will see less improvement in games unless of course the game is heavily CPU intensive.
I currently have a Q9450 and it is still good enough for my needs. I may upgrade when Ivy Bridge comes out next year. It will probably be 6%+ more powerful than Intel's current Sandy Bridge CPUs. However, if I can put off the itch to upgrade, then I will upgrade when Haswell CPUs come out in mid 2013.