Answer is simple - You should not. Really, it's just a waste of money, especially, if You have working N900.

On other situations, I would just recommend to buy N900 for 200$, and save for possible future devices (maybe some really Open one?).

Anyway, even if You're totally N9 biased, You'll probably be able to buy it for less than half price in few months - especially, after Nokia piss of fanboys by some "discounted support" decisions, while focusing on w$nphone. you can expect many fluctuating on ebay etc.

/Estel