How do I know if its time to change the host?
tower10: If your site's going down, it's probably already time.  VPS's are pretty cheap.  Alternatively, there may be some optimisations you can make to your code to reduce your concurrent MySQL connections.You can use tools like siege or Apache ab to probe the limits of your capacity, although I wouldn't run them against your shared host out of respect for the others on the machine.  You know what your capcacity is there anyway - you've already reached it.