overclocking does give a noticable difference, you only have to run a speed test to see the difference, and that is what you get from everything.
For instance, my processor is stock at 2.66GHz. I manage to get it up to 3.8 (was seeing how far i could push it) but 70 degree idle isn't my idea of fun
so i dropped it to the more suitable 3.1GHz, now it is less than 50 idle and only just gets to 60 when under stress. Now, when at 3.8 i ran superpi and went from on a 1m run it took just under 13 seconds. Compare that to the 21 seconds it takes stock and that is about a 40% increase in performance. Now as i said temperatures were an issue, that is because i am on stock cooling, but if i had better cooling it would stay as that and not lose much life as voltages are still normal and the system was stable.
It isn't just with test that you see it, my boot speeds, game frame rates, loading speeds of applications etc, all were massive reduced in time. Even at only a 3.1GHz overclock it is very stable, very cool and noticably faster.
With an i7 though it is plenty quick enough to do anything now without breaking a sweat so i would say for now atleast keep it stock and as it gets slower as technology advances squeeze a little more out of it.
You can use your old p4 systems for testing/practice/fun, would be nice to see what you could get out of them, make it a little competition for yourself. Whatever it is clocked at, see if you can get double speedes from it without overheating or losing stability. It will give you an idea of how to overclock and the results of overclocking, and how to keep a stable system and the little differences the odd tweak makes. and ofcourse it will give you that fun that you wanted