[Asterisk-Users] call center 20 seats

[Zeeshan]

hi,
 I am going to open up a call center starting with 5 and expanding to 20 
seats in 3 months. I have decided to use asterisk. I don't think I need 
FXO or any other card from digium.
If you have any document regarding setting up a call center with 
asterisk then please let me know.
What additional things I need to buy except the server (pentium 4 with 
1gb ram).
thanks in advance,
Zeeshan.