The PCI-E controller will only have so many lanes connecting to the devices. The maximum amount of lanes any one PCI-E peripheral can have is 16 (x16). A single graphics card is x16 capable and will take up the full 16 lanes for maximum bandwidth.
Two graphics cards will have to share 16 lanes between them, hence x8,x8, despite the reduced bandwidth the processing power of 2 graphics cards improves performance.
Two 460's in SLI would kick ass.