In a typical case, when I started clean, I found that WinXP uses up more RAM with new services and such.
But XP and 2k can be trimmed down quite a bit as a number of services are hardly used by a typical user. I got down to just over 65MB to 70MB, before I started to install AV and third-party firewall solutions.
I've tried Win2k, WinXP and Win2k3 (server). I personally think the initial default installation of XP is a waste of time, especially with that ridiculous candy GUI.
Win2k and 2k3 are the best in that department...Clean, straightforward, no BS GUI.