asterisk users - Oct 2006 - Load balance Asterisk server, when it is a SIP client.

I am little confused on load balancing, when asterisk server is also a sip
client.

Based on these,
XO Communications one of the largest US DID Provider, now offer SIP
Orignation Services for wholesale.
Verizon Communications One of the largest US Teleco, now offer SIP
Orignation Services.
That means no need for PRI card.
So if I take service from them, then my asterisk server will be SIP
client. Right?

How can I set up my asterisk servers so that the calls originated by
XO/Verizon goes to different asterisk servers based on load.

Has any one does this and can share that with me. Any idea or hint will be
appreciated.
Thank you,
-Kunal,