It's like Voodoo2 SLI, but less sucky
PCI Express, which is debuting within the next few months on Intel chipsets, is the replacement for both the AGP and PCI slots that are used for expansion. The speed at which data travels through PCI Express slots is a ton faster than standard PCI and AGP slots. The graphics PCI Express slots will, at their start, roughly run at the speed of an AGP 16x slot (you can only get a hold of an AGP 8X board now on high-end boards).
Graphics cards are going to fly on this new sub-system, but here's the best part: AlienWare plans on building motherboards with two PCI Express Graphics slots so you can hook the graphics cards together. This reminds me of back in the day when Voodoo2 SLI configs (two cards running in tandem) were about the best damn thing you could have in your system.
But the trick with this is that you'll be able to hook together video cards from different companies. Oh damn that would kick ass, assuming you could afford it. Meanwhile, my AMD 1ghz machine with an ATI All-in-Wonder 128 is clunking along just fine at home.