Cpu upgrade for Asus sabertooth z77

[scorchisio]

Could anyone advise what the best cpu I can fit in this is? I have an i5-3570k at minute which is showing its age.

 

[setter]

The only upgrade is to an i7 3770k. Is your cpu overclocked? Tbh the performance improvement with the newer chis isn't that much over what you have.