If you have the right equipment (and here we are talking very advanced equipment) then yes you should be able to change the 256MB to 512MB, only thing here is that it has to be the same memory (connections), and then if the N900 will take it...
In theory it should take it, just like a normal PC when you change RAM and go to the highest RAM that you MB can use.

Would be very weird if the controller on the N900 MB can't accept more then 256MB