Second monitor for 2009 Mac Pro
What is the second monitor you want to add ?
By reading configuration of your Mac Pro I find this (in the graphic card section) : 


  It has both a Mini DisplayPort and a dual-link DVI port. 


So I think you can connect two display devices on your Mac :


Your Cinema Display on the mini display port
Your other display on the DVI port


According to your second monitor, you can connect it directly on the DVI port or you should use a adapter (for example a VGA-to-DVI adpater).

Hope this helps !