I am also looking to get answers to the same question - when is a dedicated server needed? Actually to be more precise, how much power do you think I need?
I am talking with several growing clients who have lots of employees.
I want to be able to scale up to about 3000 e-mail accounts (using CPanel or Ensim).
All of the e-mail accounts would use a PHP-based webmail interface as the MAIN interface. (Probably SquirrelMail, IMP, or NeoMail). I'd like the clients to use SSL to login (and, unfortunately I guess that also means every webmail page will also go through SSL).
There won't be much else on the server. No major scripts, BBSes, and not much http except for webmail access. The only possible scripts would be something to run backups and maybe some stats.
What do you think is a good amount of power for something like this? How about bandwidth? Do you think it's too ambitious to host 3000 e-mail accounts on a single server?
This is Rackshack's $129 server:
Intel Pentium4 2.0GHz
512 MB RAM
80GB (7200RPM) Hard Drive
400 GB Monthly Transfer!
Ensim WEBppliance 3.1
When your virtual host tells you that you need to upgrade.
Honestly, most good hosts will inform you that your site is using too many resources, and recommend you upgrade to their dedicated solution if they have one. Some real bastard hosts will just disable/delete parts of your site that are using too many resources, etc.
It also depends on your current shared server. How many other sites are on it? How powerful is it, etc? Because one shared server may easily handle your site while a more crowded server may not.
I had to move to dedicated with a few of my sites about a month ago. One was a phpBB with over 3,000 members and 100,000 posts. It was just too much for the shared server, so I was asked to upgrade. Well, I didn't want to upgrade, and just got my own server. Now that site is easily within the server resources, and I even host a few other sites on it with no problem at all.