I registered to this board to hopefully make some clarifications from all of what I've read above related to Four 4870X2 video cards in a single system. Please feel free to debate the issue further if you feel that anything I say is wrong. I'm not after being right, only the truth, I've spent a great deal of time addressing this issue.
1. The chipset does not determine a motherboard's ability to support Four 4870X2 GPUs (or a total of 8 GPUs). The driver does. Of course the motherboard needs to have a minimum of Four PCIe Gen 2 x16 lanes.
2. NVIDIA did say that they tested a system with 8 GPUs (Four 4870X2s), however: "As an aside, AMD has already built a computer that has four 4870X2s in it. So it has eight GPUs; drivers will not be supporting eight GPUs at this point of time," Mr. Hook said. http://www.xbitlabs.com/news/video/d...ics_Cards.html
3. There are MANY extremely good reasons for a system to be built with Four 4870X2s. Any type of application that follows a SIMD (Single Instruction Multiple Data) design can hugely benefit from the massive parallel processing capacities such a system could provide, even despite bandwidth concurrency and latency issues that would unfold as the PCIe Gen 2.0 x16 slots auto negotiated down to a bandwidth of 4 x 4 x 4 x 4. For instance I play a role in the security community and my specialty is cryptanalysis. Such a machine could have many benefits in my field of research. The programming technique used to unlock the potential of the GPUs is called stream programming or GPGPU (General Purpose Computations on GPUs). Oh, and did I mention such a system would have a potential of 9.6 TFLOPs? (Super computer status, indeed).
4. As of this day, due to driver support among other bandwidth PCIe related issues there is no way such a system would have any advantage as a "Gaming System" (aside of course from the Two out of the Four 4870X2s that you COULD use in a CrossFireX configuration). Also, the 4870X2 cards use double wide spacing so one needs to do one of two things. Either build/buy a case that can support more than 7 extension cards (the standard) OR remove the default fan/heatsink cooling system from the video cards and install a water cooling solution which would allow for far less room being occupied.
5. There are indeed MOTHERBOARDS that support such a system. One is the MSI K9A2 Platinum. There is another made by MSI that also works (AND READ CLOSELY TO THIS PART) ... the motherboards work as far as detecting the devices within the PCIe slots, assining them they're appropriate device addresses, and auto negotiating the 4 PCIe lanes down (for this is a non operating system/driver level function). Now, as of Catalyst 8.9 (the most recent available Radeon driver) 8 GPUs (Four 4870X2s) do not seem to be supported. Now you'll notice I just said "seem". Perhaps that was an incorrect choice of words for the most compelling evidence I have to present to this point is this:
The above is ONE of my system. I have tried two different motherboards, and while I've found there is some tricky programming I can do to access each card at an extremely low level, the driver from ATI most certainly does not support this configuration. Also, I've tried every available operating system ATI supports, many it does not, and I've even spent a great deal of time dissasembling the driver in order to find a "hacked" solution... I have had no luck yet. I did contact MSI and they verified that theoretically Four 4870X2s is possible. They agree also that it is a matter of ATIs driver support. In addition, the FASTRA project which uses 8 GPUs of the NVIDIA variety uses that exact same motherboard for there stream processing super computer that they use for a field of study called Tomography. NVIDIA's drivers DO seem to support such a configuration with there CUDA development platform.
Last I would like to say that I very sincerely hope that I'm incorrect for regardless of all that I've said I'm still looking for a possible solution. In addition, the (I can't remember if I said this already): there is absolutely no guarantee that intel's X58 chipset will support Four 4870X2s (8 GPUs) but there is certainly also no reason that it would not, just as there is no reason that they theoretically couldn't work on present day motherboards with the mentioned requirements.
Anyway, I didn't mean to be abrupt or "bash" anyone and I apologize if I came off that way. I'm simply regurgitating what I have both experienced and learned to this point.