You should be able to do it: remove 1 x 256 and add 1 x 1 gig. You end up with 256 + 1024 = 1280.
I believe that Micron has a decent return policy if it doesn't work - you will have to check their web site to be sure of the terms.
edit - I seem to recall that you are in Austin. You could call Motion and confirm all the details.