I'm a bit late with this post but reviews of the Radeon HD 4870 X2 hit the web earlier today. You can find plenty of reviews on all big tech site but lets take a look at what AnandTech writes about the card:
The Radeon HD 4870 X2 is good, it continues to be the world's fastest single card solution, provided that you're running a game with CrossFire support. AMD's CF support has been quite good in our testing, scaling well in all but Assassin's Creed. Of course, that one is a doubly bitter pill for AMD when combined with the removal of DX10.1 support in the latest patch (which we did test with here). That has nothing to do with CrossFire support of course, but the lack of scaling and the fact that 4xAA has the potential to be free on AMD hardware but isn't really doesn't stack up well in that test.
In addition to being the fastest single card solution, the 4870 X2 in CrossFire is also the fastest 2 card solution at 2560x1600 in every test we ran but one (once again, Assassin's Creed). It is very important to note that 4-way CrossFire was not the fastest solution at lower than 2560x1600 in as many cases. This is generally because there is more overhead associated with 4-way CrossFire which can become the major bottle neck in performance at lower resolution. It isn't that the 4870 X2 in CrossFire is unplayable at lower resolutions, it's just a waste of money.
You can check out the full review over here. The ATI Radeon HD 4870 X2 is the fastest dual-GPU graphics card you can get but at $550 its quite expensive. The biggest drawback of the card seems to be the huge power consumption, in idle the Radeon HD 4870 X2 system used 249W and that's quite a bit more than the Radeon HD 4870's 203.7W and the GeForce GTX 280's 159.1W. In load the difference gets even larger, the dual-GPU system uses 452.1W while a GeForce GTX 280 uses 313.1W and a Radeon HD 4870 only 278.6W.