I have a 2009 Mac Pro with a ATI Radeon HD 4870 (option).
It is connected to a 2009 24" Cinema Display.
If I want to add a second monitor (non-Apple), what type of monitor/connector/cable do I need?
What is the second monitor you want to add ? By reading configuration of your Mac Pro I find this (in the graphic card section) :
So I think you can connect two display devices on your Mac :
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 !