If not you should separate users across your drives dedicated to users based on some kind of simple hashing algorithm. This is an essential service if you are serious about service. Good servers are well maintained, documented, kept up to date, and immensely popular no matter where in the world they are located. The big server ftp.funet.fi is an excellent example of this. In general this is not a question of CPU but of network bandwidth.
Size is hard to estimate, mainly it is a question of ambition and service attitudes.
Network connections would still be the most important factor. For many this is the main reason to get onto the Internet, in fact many now seem to equate the two. In addition to being network intensive there is also a fair bit of drive activity related to this, mainly regarding the caches. Keeping the cache on a separate, fast drive would be beneficial. Even better would be installing a caching proxy server.