Should I reload the OS?

I bought my iBook about two and half years ago and I haven't reloaded the OS yet. I have more or less periodically repared the permissions, but when I do intensive work on the web for example, Mail no longer sends messages. I am wondering whether it's time to reload the OS. And if I do what kind of precautions do I need to take. Would it make any difference anyway?
Thanks for your advice
Erich