500 SIP Phones calling at the same time?

How much RAM and Processor speed i need to run a PBX that allow 500 sip phones calling at the same time?
(This is for a call center)

NOTE:* This is not about space on disk. [I have lot of space, i have a Hard disk of 250GBs inside.]

1GB of RAM with 2.0 Processor?

Do you have unlimited amounts of bandwidth to allow 500 concurrent calls? You may choose to have a couple decent servers with 2 gigs of ram to lessen the hardware requirements.