send the appropriate CGI command via ethernet
That doesn't make much sense to me. CGI is a standard interface for web applications on web servers. Web servers talk HTTP which uses TCP on the transport layer while ethernet is just one possible physical layer.
My (wild) guess is, that you want to call an application on a web server with the results of the command you received on the RS485 connection.
If I'm right, an UNO is enough for your task, you also need the Ethernet shield and the RS485 module. That makes the hardware side. On the software side you have plenty of examples for the Ethernet shield (web server, web client, etc.). I never heard of Pelco before, Google tells me it might be a protocol to control PTZ camera systems but as you didn't provide that information (although should have done that) I'm guessing again. For that someone already wrote some Arduino software, although you cannot call that a library yet (http://forum.arduino.cc/index.php?topic=67590.0).