If you are just doing the internet(ie. browsing, email, etc) and some office apps, you don't even need to upgrade your cpu at all. I would add some extra ram and then think about changing your internet connection to something like cable or DSL if you don't already have it. That would give you the best performance boost right now for what you are doing.
We have crappy P166's with 32Megs of ram here at work(cheap government machines) but on a T1 so everything is really fast and there's not much of a problem until you run multiple apps, then you get bogged down with slow loads and number crunching, but just running the internet doesn't take much except a good connection and some ram if you want to do multiple browsers with email or ftp or whatever. Your cpu is adequate until you want more out of your machine.