CrossFireX is really beginning to make some strides. The technology isn't perfect, but it's a lot better than it was a few years ago. Apart from the DX11 Heaven benchmark, there was no worrying negative effects when adding the second card. The worst thing that happened was that we didn't see much of a performance gain.
The good news is if you've got some money just wanting to burn a hole in your pocket or you need something to keep you warm this Christmas, the HD 5970 CrossFireX setup is hardly a waste of money. As we see more and more driver releases come from ATI, the performance of the technology should become only better.
The ultimate setup as of today is this. The bottom line is that if you walk into a computer shop and say "I don't care about money, I just want the fastest setup", unless they're setting you up with an i7 X58 rig that carries with it HD 5970s in CrossFireX, you're not getting what you want.
Performance is only going to get better when you start to overclock the model. We've already discovered the huge amount of headroom that is available with a bump in voltages and when you start doing this to a CFX setup the performance is going to be out of this world.
A few months ago we weren't sure if ATI was going to be on top come November; GT 300 was supposed to be released and we don't doubt that the model is going to give the HD 5000 series a good fight. The simple fact is that the GT 300 isn't available. If you're getting ready to take some time off for Christmas and want to get down and dirty with some of those games that have come out this year, the chances are you're probably going to be looking at something from the red team, and to be honest with good reason.
If you want a fast setup, you buy a HD 5870. If you want a really fast setup you buy a HD 5970. If you want more performance than you can point a 30" monitor at, you get a pair of HD 5970s. And while some might say "what's the point? The numbers are so high already with a single card, why bother with a second card?", you can respond with a simple "Because I can!"
If they're looking for a bit more of an explanation, though, you can simply say that you're preparing for some future EyeFinity gaming with three 1920 x 1200 screens which pushes out 2,816,000 pixels more than a single 2560 x 1600 screen. Of course, we just need ATI to hook us up with drivers that let us use CF technology with EyeFinity.