Given we're going for a lightweight OS, I suspect lightweight applications are a good idea too.
I like the balance of Gnome Office: fully featured yet lightweight.
sudo apt-get install abiword gnumeric
This provides you with MS Office compatible Word Processing and Spreadsheet capabilities.
Obviously, if you want Openoffice.org (and have the RAM to cope), I'd suggest getting OO.org3 as detailed by Tombuntu.