If the built in video card has a max resolution of 2560x1600,
That is per monitor and you can attach two monitors, each with that resolution.
Hi Dennis, you need one of the Quadro graphics cards to do what you want to do (use Nvidia 3D output). The GTX cards will output to a secondary 3D display but only in side by side and over under. I have a GTX 480 which I use with PPro and Neo, primary monitor is 2560x1600 (non-3D) , my secondary monitor is a Sony 32" 3D TV on which I view the 3D from the timeline and FirstLight in side by side mode. To be honest, I edit and cut in 2d mode and really only use the 3d monitor for colour grading (which is different for 3D than 2D because of the need to reduce high contrast areas which cause ghosting) and for z-depth positioning for compositing and titles.
Thanks for this information. I know that collecting hardware to do 3D is easier now than earlier; but it still does not seem trivial! I can now finish selecting a machine or may end up having one assembled. I have seen several interesting posts here about doing your own.