Improve my CPU (i5 3330) or crossfire my r9 280x

I'm happy with my PC but like to gradually add or improve components. I have an ssd and cooling is great as I have a totally overkill be quiet dark rock pro 2 and after market case fans. So now I'm trying to decide wether I should get the best CPU I can or crossfire my r9 280x toxic.

The CPU isn't causing me any issues but I do wonder if it's the limiting factor now? I know this unlikely but because I'm running an older chipset I can pick up an i7 or xeons that quite significantly out perform my i5 3330. My board isn't over clock compatible as far as I know. However my cooler is significant.

My graphics card is a Sapphire R9 280x toxic. I can pick up similar 280x (not toxic editions) or 7970's for around £100-£120. My power supply is 700+ watts so I think it would be fine. Parts picker said it was. My research suggests I could see quite a bump in fps.

I game at 1080p and I like to run the odd benchmarks. I enjoy PC building and love to buy parts second hand. Which would you do and why? I'm leaning crossfire as I would love to fill up my case. Maybe as some extra cooling. However is the improvement likely to be significant? I play GTA v a lot currently.

Would a CPU make much difference to me? Should I get a Z motherboard and over clock? All ideas welcome. My budget would probably only allow me to do one of the two right now.