The actual PCI slots on the board are physically x16 slots...however.
ELECTRICALLY, they are only x8/x8 when in crossfire.
As simple as i can explain it, think of a PCI express lane as a road. More lanes in the road mean more cars can pass over it(think of cars=data). The chipset only has so many roads(PCI Express lanes), which means only so many cars(data) can pass through it. Now, if that data has to go to two different locations, it has to split the number of lanes to each card.
HOWEVER, all cards short of a 5970 or 4870x2 will have enough bandwidth not to be severely bottlnecked by the amount of data that can pass through the pci express lanes.
An 880g or 890GX based board(870 doesnt support crossfire) can do crossfire in x8/x8, and will run pretty much any card just fine. In a single card scenario, you will have the full x16 bandwidth.