If you're planning to have windows xp with service pack 2 ... then I would recommend atleast 512 mb of ram ... otherwise you can also run windows on 256mb ram fine (I have 256 mb of ram with windows xp sp1) ... but as casperjack said that you'll need another 128 mb for smooth performance.
Agreed. Running Windows XP in 256Mb of RAM works, but it's somewhat like trying to eat ice cream by sucking it up through a straw. You got a bit of a wait while the suction 'melts' the bit you're sucking on! :D