Server Powers "CPU and RAM" how many users can serve?

hello everybody ,
if i have pc with core2due 2.9 Ghz and 2GB ram how many users this server can support using freepbx ! and what is the mean thinks to make me able to determine what is the best device for my solution ,
i’ve saw many servers are atom possessors and others dual core , what if i use I7 what is the performance that i got ?

regards ,