it can be forced into a lot less...
it puts about 2gb aside for virtual memory
2gb for hibernate.sys and about another 2gb for system restore points.. it is possiblt to bring an install in at about 12gb..
the bulk of the install is the winsxs folder.. mine currently runs at about 5gb, its basically folders full of copy dlls, several in some cases but it stops all the problems you used to get with xp removing a program and suddenly finding things didnt work because it reverted to the wrong dll for them..
16gb really isnt an issue anyway.. i dont think there are many people run with a hard drive smaller than 120gb.. you can pick up a 200gb drive now for less than £30.. (ive just got a 200gb toshiba laptop drive for 29.99)
i dual boot with ubuntu btw.. but i cant use it full time as neither my DPT, graphics or movie editing packages work with it (and dont install under wine or crossover either).. ubuntu still has its problems..my inspiron graphics drivers still wont give me a decent resolution, tv support is poor, there is no real substitute for media player, and drivers can be a pain in the bum.. web browsing is badly effected by cookies as there is no linux cleaning program for distruptive cookies, its good, but it has issues..
i never actually used vista.. the laptop arrived with it, i lasted an hour before i nearly threw it across the room.. and then spent three days trying to get xp on it, because it was a laptop built for vista and there where no xp drivers from dell.. was a nightmare, but worth it.. but when i switched to 7.. i was amazed.. i never expected it to be quicker than xp.. and certainly didnt expect even a beta to be more stable..
the system tray and start/task bar as well is massively improved thumbnail preview of closed windows is great, the library system for document directories was long needed to be as good as this.. no more my documents..
quite a good review/comparison of windows/ubuntu here