I am having some frustration with this card also in regards to multiple monitors. The Asus website says: "Drive up to four HD displays (up to three with NVIDIA Surround® technology), or connect to a 3D-enabled TV for a true cinematic 3D experience"
http://www.asus.com/Graphics_Cards/GT6402GD3/#overview
But trying to do it with the included software and the Windows 7 control panel, it seems like you can't do it.
However, if you search the ASUS knowledge base, it tells you to go to the NIVIDA website, and download a driver:
http://www.nvidia.com/object/win7-winvista-32bit-257.15-beta.html (release date is May 2010--which seems pretty old to me, see below for newer drivers).
That said, the nvidia website does not specify that the driver supports the GT640 2G3D.
The ASUS website also offers these two drivers:
http://www.nvidia.com/object/win8-win7-winvista-64bit-306.23-whql-driver.html (release date is Dec. 2012)
http://www.nvidia.com/object/notebook-win7-winvista-64bit-301.42-whql-driver.html (Release date May 2012)
I have not had a chance yet to install any of these drivers on my windows 7 new build with this card, but I think I have it on another computer, which is an XP computer, and once, when I had three monitors attached, it seemed like it was giving me the option to run three monitors--but that was on a card that support NIVIDA GT 610, not 640. If you're confused, so am I.
But I will try to install a few of these setups and let you know what I get.
UPDATE: It works easily with the NIVIDA control panel software. Plug in the monitors, run the control panel software. All very easy.