Whether a person will notice depends very much on how they're using the machine, and where the bottlenecks are. I'd notice if you swapped out my CPU for a slow

Whether a person will notice depends very much on how they're using the machine, and where the bottlenecks are. I'd notice if you swapped out my CPU for a slower one or a faster one, because it's my main bottleneck in SC2. But if I was only browsing the web, I wouldn't notice. I have friends whose main performance hits are coming from hard drive access, from too little RAM (and therefore paging/swapping), from underpowered video cards, from a weak CPU, and from having too many resource-wasting processes running in the background. So, to expand on the guy's point: you might or might not notice the difference between a faster and slower CPU, because it might or might not be what's holding you back. If you're going to upgrade, make the right upgrade.