Well for crysis 2 would an i3 processor overclocked to 4.20 GHz be able to run the game on enthusiast settings? or do the extra cores of the i5 and i7 models really make all the difference? even though they have a lower clock speed.

The extra cores do make a difference, I'm not sure if C2 is quad core enhanced or not, if it is then a new sandybridge i5 or an i7 would be worth it. As for your question if an i3 @4.20GHz could run it on enthusiast, im not sure since it is the lowest class out of all the processors and is missing most of the key technologies that make the the i5s and i7s worthwhile. Also, what graphics card do you have, and how much ram do you have.