Actually, where the bottleneck is varies by the game.
Some games need a lot of CPU. For example, Starcraft 2 would be limited by your CPU most of the time. A game like Tomb Raider is probably more limited by the GPU (not that it's playable).
Also, the i3-530 only has TWO cores with hyperthreading (2C/4T) and relatively weak cores at that, so WINDOWS is going to probably eat a lot of that, especially when a demanding background task suddenly does something.
So NO, it's a big stretch to simply say a GT730 would be the bottleneck. Just not true in many (most?) situations.
So...
Again it VARIES but for very light gaming I'd get at least a GTX730.
BTW, there are different models of GT730. Some have more CUDA cores than others. Some only...