from my experience, XP really needs 512MB RAM to run well. 256MB it will run okay, but a lot of the time it i a bit sluggish, unless you disable a lot of things, such as the themes, animations, and a bunch of useless services
I have 1GB RAM in my system, and I just disable paging file completely. I also disable themes, animations, system restore, and a lot of useless services which are "supposed" to make things faster.
but yes, XP does manage RAM pretty poorly compared to OS X
mind you, it is much better than Windows 98.
Windows 98 was absolutely horrible the way it managed RAM, and systems very often crashed because of 98's poor RAM management.
I find that, properly maintained, an XP system can run quite well.
but the key words are: properly maintained.
OS X is a much lower maintenance OS than XP is. if you do not maintain an XP system, it will become very bad to run. however the way OS X manages things, given the same amount of maintenance as an XP system, will run a lot better.