What's your motherboard?
With $300 you're not going to get a whole lot extra out of your system. You could add a second 760 if your motherboard supports SLI (Must be a 990FX series), or you could move up to the 8350 and get the extra core performance. Either of those are viable options (assuming your motherboard supports the first) for better performance, though I will say moving up to the 8350 may not show that many noticeable performance increases.
If you're not adverse to switching video card vendors, you could probably get an R9 290 for $300, and that would be quite a big boost over the 760 2GB. This is honestly probably your best bet, if your power supply is up to snuff.