Depends on what you want. I run 2 usb monitors proximus p16-40921 with them. The width is a little small so I end up doing a small pic than what the cougar mfds could actually fit but it works and was fairly cheap.
Using 15 inch will add more processing work. You have to account for space added in the resolution but not displaying an image. That "blank" space takes away from processor power. It may work fine but it is something to consider.
Here is the guide for setting up multiple monitors
http://forums.eagle.ru/showthread.php?t=60815
pros: It just damn cool
Con: no sli
have to play in windowed mode