You can use upto four HD4870 X2 accelerators in tandem for an 8-GPU, 9.6 TFLOP, 8 GB graphics crunching monster. You need a 4-slot motherboard though. There are 4-slot solutions available on the AMD 790FX platform, for Intel though X48 did support 4 slots and images of prototype X48 boards with 4 slots did surface months back, there isn't such a board out yet. You do have the Skulltrail platform and upcoming X58 Bloomfield-supportive boards do promise to come in 4 slot flavours.
In a press-conference, Raja Koduri, worldwide CTO (Products Group), AMD talked to Indian website TechTree, among minor revelations such as "Fusion in 2009", here's a shocker:
"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."
Seeing how well 2xCF scales, how 4xCF scales, I don't have very high hopes for 8xCF setup. Just as with CPUs, most games are not optimized to run more than 1 thread; SLI/CF has been around for a while now, so we see nice scaling on those engines which were designed with it in mind. 4x CF/3x SLI has already proved to not scale all that much with current games.
so we'll end up needing a 8xGPU-core, 4xCPU-core setup with 2000W PSU to play games you can run on a $400 console? :/
8xGPU core sounds about as effective as NVIDIA's 3xGPU core or 4xGPU core setups. That is to say, completely useless.
8 4870's on a single board could make for one heck of a super-computer though... would be amazing to see it folding, but would be limited by even a quadcore CPU.
theoretical it's true, practical it doesn't make sense, playing games at 1280x720 at 30cm from you on a 20"+ screen will make them look blurry. Now if you move away from your monitor you will notice this less and your eyes will do the needed anti-aliasing to make everything smooth out.
problem is that to keep the image sharp at close distance on a large screen you need increased resolution, so PC gaming is destined to always have a much higher output resolution then console gaming.
upscaling either by the console or TV can be noticed when you see close by, but when you move to the proper viewing distance this effect will be minimalized.
The lack of performance scaling is more of a development issue than a hardware one. Developers are coding for the majority of gamers who don't have mult-GPU setups instead of optimizing for those who do. This makes sense since there's a much smaller number of those with than without. Hopefully, soon they'll realize that they may need to make a separate code path for those with and those without to make sure everyone is getting an experience as good as their hardware.
ATI HD4870X2 4-Way Crossfire Possible (8 GPUs!)
ATI HD4870X2 4-Way Crossfire Possible (8 GPUs!) 
