As the cores that power the HD 3870 X2 are exactly the same as those used in the HD 3870, we won't talk in detail about the features to be found therein. However, we will just reiterate that, by virtue of it using two R670s, the R680 (as the HD 3870 X2 is codenamed) features 640 stream processors split into 10 clusters of 64 and also has 32 texture units and 32 render output units (ROPs). It also incorporates all the new PowerPlay power saving features, although inevitably it still consumes about twice the power of a single HD3870 at any given load level, resulting in it being the most power hungry card we've ever tested when under full load.
Also present are all the new multimedia capabilities that were added to the HD 3870. So you get full acceleration of HD video playback and all the image enhancement techniques as well as HDCP compliant outputs so you can play protected HD content like commercial Blu-ray and HD-DVD discs on your computer.
The cores run slightly faster than on the single HD 3870, seeing an increase of 50MHz from 775MHz to 825MHz so even if Crossfire is having trouble, you should at least get an extra frame or two per second over a single card. Aside from that, though, it really is just what you'd expect from two R670s bolted together.
In contrast to the increase in core clock speed, ATI has swapped the faster GDDR4 (2.25GHz) memory of the HD 3870 and replaced it with slower GDDR3 (1.8GHz) for the X2. Although no reason was given for the switch, it's reasonable to assume that memory bandwidth isn't a limiting factor and using slower memory could save ATI some money on what is already an expensive card.
Taking a look at the PCB, we can see nestling inbetween the two graphics cores is an additional chip. This is a PCI-Express bridge that controls the communication between the two GPUs. This communication would normally be performed across the motherboards PCI-E bus, which is where all this who ha about numbers of PCI-E lanes on motherboards comes from - the more you have, the faster graphics cards can communicate. By keeping all this communication onboard, the X2 bypasses any limitations the motherboard may have.