two video cards are better than one because they make twice the noise! Twice the noise from the cards means I can hear my neighbors banging half as much!!!!!!!!!!!!!!!!!!
Hmm, two cards in SLI are definitely better than one to do NVIDIA surround with triple monitors (at the same native res/refresh rate of course). They also get you major props at LAN parties so ensure you have a clear window and some white LEDs to properly display them. Gaming chicks swoon over a guy who has an SLI setup too. =)