i3 2120 cpu bottleneck ?

depends. You see there is ALWAYS a bottleneck in your system, something will be the slowest part no matter what. So the i3 will often be a bottleneck, though you won't notice it because your fps will be really high so it doesn't matter much... but to give you an example of some titles where the i3 will bottleneck you... any game that's seriously threaded... like crysis 3 or battlefield 4. those games like "MOAR CORZ!"...

 

i will be playing games such as dota 2 , far cry 3 , tomb raider , AC4 should i not be worried and keep my i3 ?
 


i will be playing games such as dota 2 , far cry 3 , tomb raider , AC4 should i not be worried and keep my i3 ?
 


you shouldn't be worried. None of those games are seriously threaded.... even FC3 seems to top out around 3/4 cores... keep the i3, the only thing that will hurt you is when you're playing a terribly coded game like Total War: Rome 2 and the lack of the ability to overclock comes back to bite you.