As Pob said it's got 2 full independant x16 pci-e slots. Now it makes a difference depending on what you want to do. If you're just going to be using 1 graphics card you're going to get very similiar performance on both chipsets. When you crossfire though the X46 will be running x16-x16 and P45 x8-x8. This makes little or no difference when the amount of bandwidth needed for whichever application you run is very little, ie in games only at 2560x1600 does the x48 pull ahead. Although there are exceptions for example like Crysis running on Very High, the x48 will pull ahead early in resolutions such as 1280x1024.
Also the x48 boards are all enthusiast ones so the BIOS'es and settings will have lots of features to tweak and mess about with, which might help with overclocking, ie they usually have more CPU voltage choices meaning greater accuracy on what voltage you want for your overclock. In some boards you could only get the voltages in steps of 0.1v but now you can get up to 0.00625v differences. But anyway if you get a decent P45 board you'll still have those features, just be sure to check the motherboard manual online to see if it does have those settings and read, read, read those reviews on it whenever you buy
Yeah, although if the price is right they might be onto a winner. But I still think it's complicating things