Generally speaking, the performance improvement from 1st gen Core i3 to 2nd gen Core i3 is about 12%. Therefore, a Core i3-2120 @ stock speed is basically equal to a Core i3-540 clocked at 3.7GHz.
Based on what I've read you can overclock the Core i3-2120, but you have to do it the "old fashion" way by increasing the frequency assuming your motherboard allows you to. However, you cannot overclock too much because everything is tied to the frequency. An increase of perhaps 300MHz is possible before things like the USB ports are no longer responsive. Overclock way too much and you can damage the motherboard.
Therefore, it should be possible to overclock a Core i5-2120 to 3.6GHz which is basically equivalent to a Core i3-540 clocked to 4.0GHz.