Gaming on 2 cores is a no deal for newer games. You can in theory get something like a RX 460, which will most likely not run at full power (certainly not on 720P). But it will give you the most fps the cpu is able to produce.
This also depends on your PSU... Or your build in general.
Would be helpful to post the entire config (gpu, motherboard, power supply etc.)
Anyway, the i3 540 is a really old cpu, it will bottleneck most of the modern graphics card. GTX 750Ti of GT 1030 should be enough for 720p.