for windowsXP 512 is almost the standard.

for example, if you run eDonkey or expeer at background. open 4 windows for internet. and at the same time run ICQ and MSN messanger. and watch DIVX movie.. you will need at least 512 to be "smooth"

if only 256MB, windows will use some HD space as virtual ram. on notebook, HD is usually not very fast. the performance decrease is very noticeable.