I used to have 2 x 17 inch screens and a TV as my third screen for video playback. I wanted to upgrade all of this so I debated whether to get 2 x 24" or 1 x 30". The question was answered when Logic went version 8 with it's new unified window arrangement. With this new layout it made sense to have one big screen rather than 2 smaller ones that add up to around the same amount of pixels.
It has really revolutionised my workflow having one big screen for logic. One of my old 17" LCD's is still connected, and other than making a good quality video playback screen when I'm working to pictures it rarely does anything more than run a meter bridge.
I know it's not what you asked but if you did decide to go this road you would not need another monitor card as any 8 core graphics card will support up to 2 x 30" screens.I also know it's more money than a $200 graphics card and an extra screen, but there are other major benefits involved.
Kind regards