First things first! Before you even fear doing something with gparted, get the N900 on its feet again with a healthy optfs.

Here is an overview about the operations you do with Gparted-Live-CD http://maketecheasier.com/resize-cre...ted/2009/01/06 Note that your steps to just resize two partitions are even easier to apply.
If you are looking for more information: Here is a quite complete guide to gparted http://www.dedoimedo.com/computers/gparted.html