Overclocking is making your CPU and RAM run at higher frequencies than they do out of the box (go faster lol) so a 2.5GHz processor as you have picked can probably run at 3.5GHz with very little effort. This does have draw backs of course such as increased power draw and more heat generated, hence the aftermarket coolers which can quietly and effectively cool overclocked chips
As long as you don't increase the volts to the chip over the stock amount your warranty will remain in tact also.
The freezer 7 pro (as I'm using right now, is a noisey thing when its at full whack (which it pretty much has to be to cool effectively) which is why spending the extra £8 on the scythe mine is money well spent. I'm running my freezer 7 at 75% fan speed and it's still bugging the hell out of me and not doing that good a job of cooling lol
Other than that (graphics card dependent) you have a damn near silent computer there