Well I may be wrong but in my experience XP and 2K need a fair amount of RAM for themselves just to operate. Add drive cache to that and you can take away about 100 to 150MB of RAM just for the OS you are running. NT is much more moderate regarding this. As is Linux - but only if you keep it in console mode (ie no XF86, no display manager) and don't run a heap of other servers like http, ftp, proxy and so on.
Just my 0.02pp though.
