Must be remembered that the OS requires RAM, as it is windows this is a lot. MS-SQL uses Ram of course and then the server. Depending on how many people are on, depends on how many mobs are spawned - More mobs = More Memory.
As with anything on a computer, the more memory the better, of course people are normally limited by there connection also (upload normally been 256kbit) which will restrict the max number of poeple playing.
A chinese server I know of runs on a 4 way 2.8 Xeon with 4 GIG ram - but then again they have a average of 900+ people online with 16500+ account registered (handy having a 100mbit connection aint it).
Private Servers in Europe though quite low on the number of people online - remeber a 256mbit upload will hold 85 people online no problems - I've yet to see one with that many people online on a constant basis. LS when it used to run had about 56 people average online If memory serves me right - DragonNet probably got into the 50's as well.
olice: 
