The amount you will need for updates will depend on the update. PR1.1 was particularly big, and you would probably just about have got it under the wire.

I think there's quite a lot more when the n900 is completely new - all sorts of things fill it up (or empty it!) Apparently some of the Ovi store apps are a bit naughty in that respect, too. I could be wrong though - be interesting to have someone with a brand new n900 check!

If you have 47mb right now, don't worry. Worry when it gets seriously low. Or if the next update needs 60mb. Nokia will give advance warning how much it will need.

The thing is, it goes down as well as up: mine fluctuates according to what I have installed or uninstalled. Some installations free up space, by moving libraries out.