Always very iffy mixing RAM. Stick to the same type and check whether or not it has to be installed in pairs (less likely today). Otherwise you may get all sorts of operating problems.
256MB max? Chances are your system would then go above the threshhold and it would be a waste of RAM: there is an optimum for most OS's.
Cheers.
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If you're not on the edge, then you're taking up too much space.