In the simplistic example, yes - 15mhz will make a noticeable difference in OS functions and most other day to day use. Many or most things will seem improved and snappier.
If you're hoping 3D games will play leagues (or many multiples of frames) better, I'd say you'd be disappointed.
Which 3D games are we talking about? Anything that is close to the definition of modern, you're going to want an 060 processor. Skip the 040 as it runs too hot and takes more electricity = heat, unless you land one on the cheap.
Over-clocking a board and CPU like this typically results in greater heat and less stability. Be prepared to learn about cooling - if it's even an option - which is typically NOT for the average user. See if your particular CPU stamp is even capable first... then swap an oscillator (crystal) or two if it has a discrete math co-processor. Thing is, I've found the "art" of over-clocking to be a huge mixed bag. Hardly worth the risk IMO, unless you really know what you're doing and implement all the safeguards.
