ok thx guys, refer to point no 2, I understand about how hard we have to determine the specification.
I just wondering, for example : how do we know how far MySQL can handle concurrent query, and how much time ffmpeg need to transcode a single movie and how many request nginx can handle a request, I already googling for those kind of information, but not enough information on internet. Could you guys give me some clue or reference? any site has been discuss those problem? tomshardware (I don't think so)?