Bare install of win2k uses up about 55MB of RAM, 256MB leaves plenty of room for stuff like web browsing. I'm using it right now on my laptop. Even XP won't take much more than 100MB until you run other stuff on top of it. I've found that geforce and radeon drivers have a considerable memory footprint, but if you're playing 3D games then you need more than 256MB anyway.
