You have a very powerful CPU (i5-4670) that can handle the best GPUs for 1080p-resolution gaming (GTX 1060 and RX 480) without bottlenecking their performance. Of course, the upgrade would entirely depend on your budget (as well as considering upgrading the PSU to a good-quality ~500W to power such GPUs).
For me, going from a GT 640 to a GT 710 is a minimal increase in graphics power (again, may be due to your limited budget), but an increase nonetheless.
I don't know the specs of your 300W PSU. But you can get by (depending on your actual PSU's +12V and quality), a GTX 1050 or GTX 1050 Ti.