yes it is normal .
and yeah to check if your temperature is going over the limits.
try to find Savecpu app from the app manager.
what does this app do.
it allows you to choose a certain temp. which you think is a limit for your n900 .
and if the temp of your phone will increase from the one you have chosen that application will alarm you and automatically downgrade the frequency of your phone to prevent any damage .

and i dont think so you can save your battery with your phone overclocked :$