To notice it you need about ~20% increase. If it is already overclocked that much already you are aiming at 3GHz to be noticeable which will be tough.
If you are putting on the heap soon, go nuts. I would look up the records for various benchmarks using your cpu and try to beat it. If you happen to find some liquid nitrogen, try to join the 4GHz club! Keep going until you let the smoke out! The motherboard will be outdated too and so not be worth salvaging.