There are very real gains in performance at the cost of more heat produced and shortening the useful life of the chip.
Its best to think of overclocking as an enjoyable hobby if you don't already know much about it. There can be setbacks and time consuming issues to overcome.
You can easily overclock many cpu's simply by raising the frontside bus.
I've been running my 2.8 gig cpu at 3.2 gig for the past two years. Very noticable diff in performance, and I will be upgrading b/4 the cpu reaches the end of its life.