Yes, you can combine 2 Nvidia cards, even if different.
The second slot runs x4 speed, while the first slot runs at x16 speed.
I wouldn't recommend it for gaming. It should be okay for internet searching with 4 monitors. You probably want a powerful card like the Nvidia 9800 GT for dual monitors.
Tell us about your power supply. You may need a new one.
8400gs is a good card and barely uses any watts, just makesure to get one with a fan, those suckers can run hot, your power supply would more than cover it, you could even get away with less the main card does all the thinking, heck my secondary card is a x1550