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it should be ok, if your ram is the same speed (ie pc2700) if not it will just run at the slowest (pc 2100 & pc2700) it was only old edo ram that needed to be run in pairs if i remember correctly (its that far back)
it depends what your doing on your pc but i dont think there will be much noticable difference. very very old ram needed to be run in pairs and new "dual channel" ram needs to be run in pairs on a dual channel motherboard in order to get the increased speed that dual channel gives you but you can run dual channel ram on its own aswell.
like i said i dont think there will be a noticable difference between 1gb and 1.2gb of ram, so you could sell the 256mb stick or just leave it in your pc as its not doing you any harm.
sorry misread your post.
from 256mb to 1.256gb will be a huge difference
windows should load quicker. progorams should open faster, you can have more programs open at the same time and will also be faster. your pc should feel more responsive. programs and games should run smoother too.