One 1gig will always give you 1gig no matter what phone you have. I have 1 gig flat rate too... and i have only surpassed it once. So i think you will be safe on that end. If you have a choice then pick a lesser quality video. It will save you much.. normal surf will not be a problem.

Also.. it will take a month or so before you will understand the device. The first thing is battery usage. If you don't think about this.. you will be on charger all the time. You could get up to 3 days if used correctly.

The community will long surpass Nokia in the life cycle.. so i would not be bothered about that. The n900 will give you more power than any other device for it's price. Also if Maemo is not good for you.. you could chose other OS's. So you'll be busy 24 month from now guaranteed..