I think you're likely to find a package for this in the Debian world that you could get working on the N900 using the Easy Debian environment that's been mentioned many times on forum...

Or there's always the simpler and non-technical solution: set up an automatic transfer of funds from your main checking/current account into a savings account, and forget it's there until you need it. Doing this smooths things like my yearly car insurance payment out nicely for me.