Just bought an X51 with Nvidia 640 card. On the back of the nvidia card, there're 3 connectors, DVI, HDMI and display port (not sure).
I have 2 monitors and here's my setup
nvidia HDMI -- HDMI 2 DVI cable -- DVI on display 1
nvidia DVI -- DVI 2 VGA cable -- VGA on display 2 (8 year old VGA analog monitor)
If only 1 is connected, boots fine, if only 2 is connected, boots fine. But when both are connected, only the DVI one will work on display 2, and in windows 8 Nvidia software does not see display 1 nor can it detect another display at all.
in BIOS, graphics is set is discrete, not IGFX.