could i have two asus gt640( i already have one) in my pc, one for my main montor, which is for gaming, and another for my two other monitor which i use only when on my desktop for multitasking, or am i better getting another graphics card? can i run 2 different cards at the same time?
You can run two video cards at the same time but you are restricted to the one operating system at a time so what's going to be on the other screens will be the same desktop or if you have the option to extend the desk top then you could have what amounts to the desktop spread out over the three screens. The Nvidia control panel would have to have that option to use , if you don't have that option then you screens will all show the same thing and the mouse won't travel between the screens.
Ultimately the goal is to have the extended desktop so the mouse will travel in between and you can have one screen with the game running and the other screens with different programs open, the game will have to be set to windowed mode so it's only on one screen.
It all depends on the 640 and if it supports that feature.