With todays chip speeds theres very little gain in overclocking. So you get a few more frames in Quake 3 ( not that you would ever notice in playing the game). In applications you really cant see the difference.
I have an XP 1600. If I managed a 10% overclock ( which most of the time is all thats realistic) I'd have the equivalant of an XP 1700. WOW!!!
If you want to overclock higher than that then you have to start buying exoctic coolers, more fans etc. By the time you've paid for this lot you might as well have bought a new chip.