Definitely CPU, the RAM is good enough for any gaming build. Check out FX-8320, or switch to Intel and go for 15-4690k (assuming you want to overclock, otherwise i5-4460)
if your not a hardcore gamer you don't need to change anything !
I concur. If your machine is feeling a little sluggish, spend 50 bucks and get a boot SSD if you don't have one. Will make the biggest impact. Assuming you don't have one.