Two is, generally speaking, better than one. At least in the computing world, where the more chips and such you have the faster you'll be running. That's the idea behind AMDs new ATI Radeon HD 3870 X2, which has not one but two chips inside for your graphic-computing pleasure.
As games, such as the insanely-demanding Crysis, get more and more sophisticated, PC gamers are forced to cram more and more firepower into their towers, including $450 cards like the 3870 X2. Of course, Crysis is so insane that there isn't a graphics card out there yet that can play it at full specs, despite it being out for three months. Is that the fault of over-zealous game producers or lazy graphics card manufacturers? You decide.