For heavy gaming, it really depends on the game. Overall the i5 will probably be faster in most games. There are a few where the 8320 would win though. You can overclock the 8320 a little easier, but keep in mind it uses more power and generates more heat so your cooling system has to be better.
A far better option than both is the i5-4670k, but only if the budget would allow it without sacrificing video card performance.