In my opinion overclocking is just a waste of time and a sure way of reducing the life time of your CPU and possibly even your VGA card and RAM. The real bottleneck in modern computers is the hard drive and RAM. Memory is so cheap now it's crazy to spend a load of money on a more expensive processor when you can give your system a real boost by adding another gigabyte of memory at a fraction of the cost.
So If I don't believe in overclocking why did I just overclock my own machine just now? Well I often run virtual machines using QEMU. My E6400 Core 2 Duo processor has the vmx flag, so that means the guest operating system runs almost as fast as it would run natively. I have been playing around with boot times as well and found that a Fedora 9 i386 guest boots up in 44 seconds (using bootchart).
I wondered if I could improve the speed of the guest by overclocking the host CPU. I was disappointed. The boot up time actually increased by 10 seconds! You can't of course rely on a single measurement because processes running on the host system take a slice of the CPU and also compete to read and write from memory and the hard drives. So I ran the test again. The second time around, the boot up process clocked in at 50 seconds.
I can only guess that the increase in core temperature caused by the overclocking, results in the less overall efficiency. You would say, I should download one of those number crunching benchmark suites out there and see what figures they produce but those results would be completely irrelevant. I don't do number crunching. The most processor intensive task that I do is running qemu. If Qemu doesn't get faster with an overclocked CPU it's no use to me.
Running Netbeans and building java applets and applications is also heavy on the processor but then, it's heavy on the hard drive and the RAM as well. It's hard to measure performance improvements with these task but it doesn't feel any faster with a souped up processor.