The thing with laptops is the confined space. I don't know the specific measurements but you may get away with an upgrade to the gtx 680m. As for crossfire, I do not know for sure but there it is a pretty good chance that it wouldn't be possible.
As with almost every gaming laptop, cooling is an issue. Almost all of the gaming laptops I have used needed a cooling pad under.
Another thing to note is that often, in consumer laptops like these, the wattage of the included PSU's is only enough for the original components included. Even if it is constantly plugged in, there may not be enough juice to do the xFire or even the gtx 680 (the gtx 670 is more power efficient than the gtx 680).