Yes it has two PCI-E x16 slots and supports CrossFire. I've got the P5E myself (only wanted DDR2 as by the time DDR3 costs sensible money, it'll be upgrade time again anyway) and it's an awesome board - I think it's safe to assume the Maximus Extreme is pretty much on the same lines.
However, "futureproof" isn't quite possible. After the Penryn Core 2s, Intel will be ditching LGA775 for a new socket and a new generation of CPUs (Nehalem) so bear in mind that even today's most expensive boards won't have much of an upgrade path at the end of 2008. Yet another reason why I personally think a cheap DDR2 setup is the way to go for today.
Then again, if you keep waiting for the next best thing, you'll always be waiting so it comes to a point where you just have to go ahead and purchase. Just don't think that anything you buy today is going to be "futureproof".