720P and Low settings, is not the correct way to see potential cpu bottlenecks.
The reason being that Low presets, tend to also lower settings that can greatly affect CPU calculations i.e. distance settings. All in all, low settings means also lower load for the cpu for many cases.
You actually need ultra settings at 720p, to see what the cpu can do. Now it is possible, that a card like a 650ti will have a hard time with Ultra settings even at 720p. In such case, the extraction of a safe conclusion, is not possible. Or in order to be possible, it will require a lot of fine tuning from the user, in order to really lower only the settings that affect only the gpu.
There is however a very simple way to see what you will get. Go to youtube, type the cpu you want, the gpu you want and the game you want. profit!