If you Crossfire or SLI, chances are you'll run in X8 mode. I read a review somewhere, looking for it, and it seems that PCI-E 2.0 X8 (PCI-E 1.0 X16) mode is only a bottleneck for the latest GPUs like the 5870, 5970, etc. Otherwise, then you're fine.
Sorry for the nooby question, but you guys are probably used to it. Are there any advantages running two graphics cards in x8 x8?
There's no advantage to it - it's simply a necessity sometimes. The average motherboard simply doesn't have enough PCI-E data lanes to give each GPU a full 16 lanes, so it divides them between the devices evenly. Thus, even though you may be sticking each GPU into a "x16" slot, each slot only gets eight lanes of data bandwidth to use.
There are exceptions to this - some motherboard chipsets have enough lanes to give each GPU a full 16 lanes, the most recent example being Intel's X58 chipset. But as already mentioned, having only an x8 pathway usually isn't a serious issue, except with the most powerful cards (i.e. AMD's 5800-series cards).
i7 2600K | ASUS Maximus IV GENE-Z | GTX Titan | Corsair DDR3-2133