I am a big fan of the overclocking wiki page. Even if you choose to keep the stock kernel, there is some good info on how you might keep the N900 in a low power mode: ignoring nice processes and declaring everything you can as a nice process, for example

I know there is a way to do the following, but I don't know what it is. Why can't I underclock and undervoltage everything on the phone when I am not using it -- when the screen isn't on, and then put it to some other setting when the N900 is in use. Can the N900 detect when I'm overwhelming a EDGE network and bump me up to 3G, and then drop back down to EDGE when the network traffic slows down?

What else can we declare as a background process? I would like to make everything possible a background process, but there are a few holdouts: modest, the address book, updating contacts, none of these really need to be processed in real time, but seem to slow down my device whenever a new network connection is established.