What are the hardware required for Call Center ACD?

i want to build a call center and i have choosed your system “Call Center ACD | Asterisk” for this purpose.
but i want to know what should i have of the “hardware required” to build this call center.
i am a Linux administrator and can config Asterisk system.
i am having 4 seats (Desktop Computer ) and Two analog telephone line.

i hope you help me to select all hardware required and where i can get it.

If you are using a PSTN line to receive the calls for your call center, then you need a PCI card and depends on your line either digital or analog you would choose and then further.
If you are using a pure VOIP like SIP trunk etc then you only need a server with good specification and no additional hardware is required other then client sip/iax phones.