Buying a new CPU

First, look up what kind of i5 CPUs are compatible with your Motherboard (not everyone may be accepted), check if they are compatible with your RAM, and then pick up the one with the highest amount of GHz that doesn't surpass 150 euros.

Posting your whole system would be helpful. What's your current specs? Please don't forget to list the PSU, too.

Current CPU: Pentium(R) Dual-Core CPU E5700 @ 3.00GHz
PSU: Will be upgraded according to CPU needs
Motherboard: Will be upgraded according to CPU needs
GPU: Nvidia Geforce Gtx 650
RAM: 4 Gb DDR3
OS: Windows 7 64 Bit

Can't you add a tiny bit more

The i5-4670 is very good, and overclocking is never needed. It only boosts benchmarks and a handful of frames in certain games.

I'd seriously recommed going with an amd cpu. I just bought the amd fx6350 six core at 3,9Ghz (overclocked 4,2).
I had the chance to try it out at a buddy's home and everything runs like a charm. and it's only about 100 euros

For gaming only i5 is indeed more than enough, I would choose between these 3

i5 4460 - 160 euro

i5 4590 - 175 euro

i5 4690 - 200 euro

I also recommend upgrading your gpu to a gtx 760 or better or r9 270x