All intel non-K cpus can be overclocked to 400mhz more
than their turbo boost. Of course they need a P67 or a Z68 mobo.
Here you can see a few tips how guru 3d overclocked an non K processor: Core i7 3820 processor review
And here are a few things about you processor from intel: core i5 2300
So you can see that you can achieve something like 3,1+0,4=3,5Ghz and this is only possible by messin with the multiplier and you can gain some more by messin with BCLK frequency.
This is also an article for non-K from xbitlabs: Intel Core i5-2500, Core i5-2400 and Core i5-2300 CPU Review
But in the end it shouldnt matter a lot to gain a couple of FPS if you only play games as some other people said...
Edit: Some info from tom's as well: Overclocking: Sandy Bridge Changes The Game
Edit 2: :P
getting a prebuild watercooling as in the corsair h100 be worth getting at all
I would agree with this, in my opinion such small o/c isn't something air cannot do...