Honestly, you're probably going to have to save up some more money before you consider an upgrade. An upgrade to a modern platform also requires you buy new RAM, as nothing new uses DDR3. In terms of new stuff, maybe you could get something like an Athlon 200GE, a low end motherboard, and 8GB of DDR4 within that budget, but that setup would at best offer similar performance and at worst be a slight downgrade over what you have.
The only other option within that budget would be to look at some older Intel chips that you can use your RAM with eg. see if you can grab an old Haswell based i7 eg. Core i7 4770k or 4790k and a motherboard used.
In any case, you are probably more GPU bound when it comes to gaming, if you absolutely...