I have always installed two but I'm starting a new build after a couple of years and wondering if you guys think the cards of today can handle two large monitors. I do often use two monitors together. One for mostly photoshop, dreamweaver or gaming and the other purely for reference, email, etc. In the past I thought I would use SLI more but it was kind of a nuisance to setup and discontinue so more often then not I would just run the game on a single card.
If you are just looking for more desktop space almost all new cards can accommodate 2 displays no problem. If they these are high resolution, meaning over 1920x1200, I would just double check the video card specs. If you are doing 3d, gaming or rendering, on more than one display you may want to get something that can handle that load. But even the low end cards will handle 2 displays in a desktop scenario.