Ok. What I reccomend you do before taking my word that it's the processor and forking over a couple hundred dollars for a new one is to set up some performance counters in windows. These will tell you how bogged down certain parts of your system are.
This is streight from microsoft about how to set up performance coutners.http://support.microsoft.com/kb/305610
Here is some good info about which counters to set, and what they are telling you about your system.http://www.windowsnetworking.com/articles_tutorials/Key-Performance-Monitor-Counters.html
Let it run in the background while you work/play and analyse them after a couple hours or whenever you're done.
Lastly, please don't just turn all the available counters on. Choose the ones that you want from the second link, otherwise your system resources will be killed just from monitoring.
Hope that helps.