Hello Anonymous. 3 ghz is a pretty good speed to have. If you just use your computer for everyday use, then you really won't need that much. Part of it really comes down to how much memory you have(RAM), and the other internal stats of the chip. Example, How much and what level of Cache. FSB mhz and so on.
However if you do game or use your pc for high end video and photo editing, you may want to kick up the juice a bit(ex. cpu with more cache, and fsb as well as higher clock speed).
For most, everyday activities on a computer(and even some gaming) a 3ghz single or dual core processor will be faster than a 2.5 ghz dual or quad. Mainly because most programs out there aren't able to utilize the additional processing cores. Pair the cpu with a 2-4 gigs of ram and you should be okay(more is always nice too)!
For more info, you may want to check out wikipedia.com and look up things like Cache, FSB(Front Side Bus), Core, Hyper Threading(HT) etc.
But in the end, anything between 2.0ghz and 3.0ghz will do you fine. It really comes down to what you plan to do with the computer. If you think you'll need more computing power, step up to a dual or quad core(i3, i5, i7 for example). Ask your local computer professional for more details.
In my opinion though, if you're building a computer go ahead and get one of the new i3, i5 or i7 cpus. They're pretty much going to beat the pants out of the other Intel chips(and most AMD). Not to mention, you'll have plenty of power for just about anything you can throw at your computer.