It sounds like you need more RAM. Changing your processor won't make much of a difference if you don't upgrade your RAM. Your RAM is a buffer. Think of it as a funnel. Imagine having a very fast processor pusing lots of data through a tiny hole. If you upgrade the RAM, it'll open up the hole a bit.
You can also try stopping of some of your unnecessary background apps by going through ctrl-alt-del or msconfig. See if that helps, too.
If all fails, then perhaps upgrading the CPU will do the trick.