You can't really use the internal memory (the 32gb eMMC) as RAM. 768mb of it are used as swap by default, and frankly unless you really need to keep lots of applications open in the background it won't help you to make it bigger because it's incredibly slow by comparison to ram. You certainly can't use it as video ram, and the idea of needing 1gb video ram with the gpu in the N900 is ridiculous anyway.

If on the other hand what you want to do is make more space to install applications, there is a thread here about doing that http://talk.maemo.org/showthread.php?t=35122 with step by step instructions in, though any tutorial about resizing linux partitions will do, if you can find fdisk for the N900 somewhere.
If you want a separate partition for easy debian, why not format (part of) your sd card with ext3 and put it on there and leave the n900 storage alone?