It always depends on the application (game) and the resolution used. Generally (with some rare exception, less then 10 percent today) games tend to become so GPU-limited as the resolution increases, that an i3 or an i5 only makes 5fps difference, which means that a better graphics card delivers more most of the times. The SB/IB Intel CPU's are truly awesome, and an i3 will do very well in most of the games, especially if you play games in 1080p or above.
However, there are already games out there with decent quad core support, not to mention that this trend will become much more dominant in the future, so an i5 would be more future proof for sure, and again, it really depends on the games you are planning to play. If you want to play games like Planetside 2 or GTA (etc), an i3 won't be enough, me thinks.