ok, first flash your device with pr 1.3 - Latest nokia firmware
then flash the kernel with kernel-power - much better kernel than the stock with loads of goodies. (latest version on -devel repo)
then install CSSU - Could define this as a sort of a service pack with updated/modified packages that will enhance the device capabilities.

you should be then having your n900 upto date,

also you could enable extras devel/testing and extras repositories for applications downloads/updates.

Its always better to read well about each of the above i mentioned, as they have their pros and cons.

hope this helps.