Round Robin IP Assignment

My company sends XML request to Amazon but Amazon will only allow 1 request per second per IP address. They (amazon) suggested I send each request using a new IP address within a range of addresses. My datacenter has no problems about giving me more IP’s. I just dont know how to setup my firewall or server to allow me to do this. I am running windows 2000 servers and my firewall is a Watchguard Firebox III 1000. Any suggestions?

thanks, Jeff