The first thing I have to say is that it's nice to see CrossfireX with the HD 4870s not having the same issues as we experienced with the HD 4850 setup. Adding three cards into the system, while for the most part did nothing, it didn't offer horrendously backwards performance, bar one or two tests.
The numbers here today do have a big up on our 3GHz test bed, and it shows that you do begin to hit CPU limitation in some areas with such a massive amount of GPU power. At 3GHz we find 3DMark06 sits around the 16k mark; here today our CrossfireX configuration scored over 22k.
The good news is that 4GHz isn't exactly out of reach for a lot of people these days; it's one thing to turn around and test Crossfire on a 5.7GHz LN2 cooled system, but that doesn't help 99.9% of computer users out there. 4GHz shouldn't be too much of an effort with the right CPU and extremely good air cooling, or some decent water cooling like we used here today would certainly suffice.
The HD 4870 is an impressive piece of kit and does a good job of outperforming the GTX 280 when in Crossfire configuration, providing the game makes use of the technology, that is. Ultimately, the GTX 280 is the fastest single core GPU on the market at the moment; but that's not the be all and end all these days. Sure, if you've got money to burn, buy one; buy two! - But truth be told; the everyman and women who wants a good gaming experience and doesn't want to break the bank is going to be looking at the HD 4870 priced at 1/2 of the GTX 280. If they really feel like indulging and the games they play make use of CF technology, purchasing a second will put them at the same price as a GTX 280, and mostly with better performance.
AMD are now offering extreme performance at a very good price; NVIDIA on the other hand offer extremer (is that even a word?) performance at a higher price. Is the extra cost justified? - That's really up to you. Which route you go is going to come down to how much money you're willing to burn. Throw in the cost of a 780i or 790i over an X48 and going SLI becomes very expensive, very fast. But we're not na´ve; the ultimate gamer with no budget is still no doubt going to be looking at SLI, as it's ultimately the fastest solution.