Not sure you'll get 4.2GHz out of it on that cooler, but you can definitely overclock it. Each CPU is different so you'll only find out how far you can push yours once you try. I'd expect around 3.5GHz would be doable, possibly more (That is just a stab in the dark based on how most chips seem to OC, I have a similar combo - Xeon X3440/Asus P7P55/Hyper 212 laying around but haven't gotten around to overclocking it yet)
Kokolin :
No, you cannot overclock a non-k i5
Not true for the 1st gen Lynnfield/Clarkdale chips. While non-K chips are multiplier locked, the 133MHz BCLK on 5-series chipsets (P55/H55/H57) wasn't tied to everything else like it was on Sandy Bridge and later, and could still be overclocked to around 250MHz