IT all depends on what your server is being used for, if it's all html and pics and now scripts 75k uniques is the max before I'd start a new server, I have some php/SQL heay scripts on sites that need a new server at about 25k uniques.
If your load stays below 3.0 you should be fine, if it's above 3 and it's slowing things down, toss another server at it
