If all you do is office stuff, surf the web and email the difference will not be noticeable. If you are a hardcore gamer, there will be a slight reduction in performance.
It should be noted that when it comes to games the video card and RAM play a large role in performance as well. A 2.4 ghz system with 2gb RAM and a 512mb video card will run circles around a 2.8 ghz machine with 512mb RAM and a cheapo video card. Its not all about CPU speed.