What do you wish to do?
Before I make recommendation, could you give us little info about your system. what motherboard do you have, what power supply is in use, what storage configuration.
If you want more cpu for rendering and other cpu demanding application then I would buy an i5; this way you will save money instead of buying a mobo + a cpu but only the cpu. If you are not going to overclock take the i5-3350P if you live in US as it's the cheapest i5; you can overclock this 400MHz if you have the right chipset and motherboard supporting this kind of feature.
If you're more into gaming then put the money on a decent gpu, at the moment 7950 and 7970 are great choices but if nvidia is more preferable then the 760 and 770 is what you should look at as they went down in price making them interesting alternatives.
Aside from the hardware try to see what you don't need and uninstall them, get a application that shuts down services and background activities, such as Razer Game Booster. I also suggest you read these
tips
I hope this helps.