Afaik you can have a lot of files on your N900 and it would still be pretty fast... It's just that when you start installing alot of apps and do different things like enabling the devel repo... etc. that's when you start feeling the difference... But you can still can a do couple of things to have it go faster.. Like using swappolube and many other solutions that's available to us.

Not sure what you exactly mean by the 2nd question.. So can't help u there..