[sqwebmail] Number Of Users Guideline?

[rkl]

Are there guideline for the number of users on a email system consisting of 
sqwebmail/qmail/linux/intel? 

Also, how much diff on number of users for hardware as below: 

intel 2.4 w/2G ram =
intel 2.4 w/4G ram =
intel 3.x w/2G ram =
intel 3.x w/4G ram = 

thanks
-rkl