Hello! I have an i5 2500K that is currently NOT overclocked. If I WERE to overclocked it from 3.3 to, day, 4.0 ghz, would I notice a difference in performance? If so, where will I notice an improvement e.g. video playing, web browsing, downloads, ect?
Also, I have been planning on buying an i5 3570K for my second computer. I WILL be buying a CPU cooler if it is worth overclocking. Is the 3570 a good choice for a CPU? I see really no difference between it and the 2500K besides it being an Ivy bridge and not sandy bridge CPU.
For general, everyday usage, no, OC'ing isn't really noticeable, no matter how far you go.
You have to realize that most everyday tasks don't even need the stock 3.3 to be plenty fast and as responsive as it would be OC'd to "whatever". You'd NEVER notice it.
OC'ing benefits gaming and other CPU intensive tasks, generally (none of those include what you mentioned).
That's not to say that you shouldn't at least try it for yourself (with the proper cooling), just to see it for yourself, but if what you describe is literally all you do, an i3 (or even a high end Pentium G) would be plenty for you, honestly.
Now, if you plan on doing some high end gaming or video encoding, you'll definitely see the benefit.
As far as a 2500K vs. a 3570K goes, the 3570K is a bit faster overall (5-10%), but for your uses that you mention, that wouldn't even be close to noticeable.
If your goal is to test out the OC'ing waters, get another 2500K, as it's runs much cooler than a 3570K when OC'd.