Stick with the trusty 2500k...
You need at least 250$ for motherboard/cpu/ram for a new R1200 build and it's going to perform about the same.
Sadly your motherboard doesn't support overclocking so you're kinda stuck until you have a bit more to spend.
What games do you play and at what resolution? It might be more sensible to upgrade your graphics card, depending on what you have now.