bit of a dilemma this... the cpu is relatively weak and is likely to hold back a stronger gpu. limiting its fps...
but its the gpu that would bring the highest return in frame rates, if its replaced.
buying a new amd cpu atm isnt a good idea unless you can get an 8350 dirt cheap (and your motherboard supports it) as there new chips will be out later this year.
buying a new gpu isnt a good idea either as amd's new polaris is launching in a couple of months so the price of current gen gpus will likely drop a bit more in response to retailers dumping old stock in favour of new . which means, possibly more options for the money...