Rationale :
If you want to upgrade, an Athlon II x4 645 would be the most direct increase in power for about $70. However, it may not be possible to find one new anymore.
Would i find an increase of Performance in Fps for games like Far Cry 3, Tomb Raider and Battlefield : Bad Company 2 if i upgrade my Cpu? I've read somewhere that a Vishera Fx-6300 is a good price/performance CPU and it's affordable, i just need to find a store that sells it in my country though.
My Current Set-up :
Athlon II x2 240 2.8 Ghz
Kingston 1x2 gb DDR2 800
AsRock N68c-Gs Fx Mobo
Sapphire Radeon R7-250 1gb Gddr5
WD 500 GB Sata HDD