WHOO. JS tossing my site around in examples of high traffic. Im so proud.
http://www.aquariumadvice.com/awstat....aquarium.html
Above you can see a link to my live websats as of midnight. I really want to shy away from giving you a number as far as the number of websites of this kind of traffic a given server would support because there are so many vairables. I am on the Power 2600 server from SB and this site is basicly all alone on that server. Our server load will hover around .10-.20 depending on number of visitors, etc.
I would guess that you would not see a performance drop until you had 5 or more sites of that load on a single box. To many high load sites and you will find yourself in a CPU crunch or an ISO crunch for disk access times. Thus the overall load times of pages will begin to lenghen. Of course if you had only one or two larger traffic sites you could fill the box up with some smaller traffic sites aswell.
There is a point that splitting the database and webserver aspects apart into seperate box's makes sense. AquariumAdvice is not at that point and probably wont be there for a while.
When sizing the box you need to think about the kind of traffic you will want it to support. Is this box going to be for a hosting venture or for a website you have or plan on having? What kind of site will it be? database driven? Static? Multimedia? What is your perceved number of visits and visitors per day? What kind of page views do you expect to see? All of these will factor in the overall performance of one box over another.