The ALX system is Alienware's moniker for PCs that will have the Graphics Array. As you can imagine, putting two high-end graphics cards into the same machine will create some heat and power problems. Alienware solves the heat problem by including water cooling with all of their ALX systems.
Modern graphics cards draw insane amounts of electricity, so each ALX system will have an 800 watt power supply. This may increase to 1000-1200 watts as your needs expand. I don't want to see that electricity bill, but if you can afford the Alienware, you can afford the electricity.
Alienware had to develop their own software in order to tie all of the hardware together. They have written a Windows-only driver that will assist in combining the video signals from the two graphics card into one signal. You can imagine the difficulty in getting everything to synchronize correctly, but Alienware seems to have passed the initial hurdle with its working model.
The software does 50/50 rendering, which means that one video card draws the top half of the screen, and the other card draws the bottom half. Of course you may have noticed, in modern games, the graphics complexity is not distributed evenly across the screen. If you look at games that show a lot of trees, grass and other vegetation - you will realize that the bottom half of the screen has a higher polygon count than the top. Alienware has a solution for that as well.
Now that Alienware has successfully made their Graphics Array draw the two halves of the screen, their developers will now make "Graphical Load Balancing." The software will predict what part of the screen requires the most processing power, and then assign the card to render that part. This will split the processing power evenly between the two graphics cards. If you are playing Far Cry, there will be times when the "bottom" graphics card may only be rendering the bottom quarter of the screen because of the lush vegetation.