Those specs should be more than enough to play overwatch and most games should be playable. People are getting 60 fps low on Overwatch with pentium g850 and gt 730 last i saw, lol...
Probably your OS is just a complete mess. Could be thermal throttling as well considering the age, ESPECIALLY if you smoke near it. if I were you I'd get in there with a can of compressed air and just blast everything (be sure not to tip the can and hold the fans near where you're blasting still or they can spin really god damn fast and generate a damaging charge)
If you're set on upgrading though, grab an SSD, that + a clean install can do wonders. Nothing else is going to net you more performance for that price range you've set, it's a really big upgrade don't underestimate it.
Your current gpu/cpu setup is fairly well balanced as is so I don't think it's worth upgrading just one of them.
This post was edited by DCSS on Apr 29 2017 03:23pm