You only really need one card unless you plan on doing an eyefinity type setup (gaming on them all). Most GPUs can easily render desktop space on 2 extra monitors while gaming on the primary without breaking a sweat. The primary concern is having all the right connectors (keep in mind DVI-D cannot convert to vga).
If you do want to eyefinity, you'll likely need some display port monitors to use, I think. However I'm not entirely sure you can even use multiple resolutions in an eyefinity array (I would expect it to need to form a rectangle, if you have gaps missing, you may lose pieces of the game image).
Can't go with eyefinity...
I need cuda cores for primary gpu. Limited to Nvida for primary GPU.
Not for gaming, but other applications (photoshop, internet browser, dvd, premiere, excel, matlab, word ----all open at same time..)
Looking to create a system with at least 5 monitors. Not to important if applications can't stench across entire setup. I've used different cables and adapters before, so that's not too much of a concern.
Would a gtx 680 and a gt 640 work well for 5-6 monitor setup?