What are you doing with your machine that's so cpu intensive? I used a 600MHz machine for years as a software developer and never had the slightest problem with it, it flew through everything I needed to do. I have a 1GHz machine now and it's a lot slower because this company forces us to use antivirus software that scans every single file in real time. Unless you're doing rendering or really intensive computations all day, you just don't need a faster machine. I would say that for most people, it's very difficult to justify more than P3-500 if you're only using the computer to do your actual work. Maybe RAM is the problem as others have suggested, just open all your programs and view your memory usage.
Now if the machine performs slowly because it isn't properly maintained, well that's a different issue.