During normal use you are not going to feel a difference in speed if you overclock a PC. Most systems will only gain around 400Mghz before it won't post. Some folks have successfully gained 600Mghz to 1000Mghz but it depends on the motherboard, RAM, and processor. Of course, some very serious cooling also.
It's been my experience, that if you can double the speed of the processor, by either overclocking or upgrading, you will feel a difference with normal use.
I have, at one time, overclocked every PC I have owned by 600Mghz at least (I do run some serious cooling) and I have found that it didn't feel any different from stock speeds. I have always resorted to setting them back to stock speeds.