Let's say that u want play GTA IV or Crysis without CPU...oh. wait, U can't (maybe in the future)
Now, the i3 is a solid processor but u can't compare it with the i5 or i7 since both are quad core CPUs.
U can take a look of this articles and see what happen with 2, 3 and 4 cores.
1- Is Intel's Core i3-530 Fast Enough For Performance Gaming?
2- Part 1: Building A Balanced Gaming PC
for Intel rigs.
3- Part 2: Building A Balanced Gaming PC
for AMD rigs.
As u can see in the articles, the performance depend of the CPU and GPU not only from one of both. A high end CPU (980x six core) with a low end GPU (ATI 4650) is a bottleneck, or a low end CPU (Pentium IV) with a high end GPU (ATI 5970) is another bottleneck.