Depending on what you use your computer for, is what you should do with the memory.
If you just play games and surf the net - then you'll get the most out of having the
RAM running faster (so remove the 512mb and replace it with the 256mb).
However, if you do more development (video/software editing) where your computer crunches alot of numbers, leave the 512mb in there...as the more memory will definatly be beneficial.
If you do a mix of both, i'd leave the 512mb in there still...same reason.