Well the real question to ask is what will the new CPU be used for?
Are you primarily gaming? If so I would stick with what you have because most games are GPU bound with a few exceptions such as BF3, Starcraft2, and Skyrim just to name a few. But even in these games the performance gain of a better CPU is not enough to justify rebuilding your rig, to me at least.
Now if the CPU is going to be used for video editing/authoring, number crunching, folding, or anything else along those lines, then yes, it is a definate performance gain that is worth it.
Also, you could overclock your current CPU if you haven't already. Give it a bump up for some new life until you run it into the ground. Then upgrade