the 2120 is a great gaming CPU. i just upgraded from it myself and that was work related and not gaming.Undervolting i3 2120 (the only "fun" there is)
<-- some stuff i did when i first got it.
as far as performing as well as an i5 when gaming; depends on the game and the GPU. for instance take BF3. in single player campaign it does not matter if its an i3, i5, i7 becasue that is GPU bound and any of them will perform the same. but in multi player maps where there is a more substantial load on the CPU a lower end or mid range card will not choke the i3 and it will be fine but a higher end card, because of higher settings and frame rate, will cause the i3 to not perform as nearly as well as a quad core i5.