For games that only use 2 cores, the Core i3 is fine. However, for games that can take advantage of more than 2 core the Core i5 will be better. Crysis 3 is an extreme example of a game that runs much better on a Core i5, than a Core i3. I believe the difference is 40% higher FPS with the Core i5.
As stated, Crysis 3 is an extreme example. Other CPU demanding games that can use more than 2 cores (like Hitman:Absolution) typically gains about a 15% performance boost with a Core i5 compared to a Core i3. Still very playable though.
If you have the money buy a Core i5. If you don't then buy the Core i3 or consider a quad core FX-4xxx CPU or a 6 core FX-6xxx CPU.