Did the math.
Originally Posted by tpi2007
Hmmm, so I presume they got acceptable performance for CPUs with only 16x PCIe 2.0 lanes where each card will only get 8 PCIe lanes of bandwidth.
- IB and Haswell have PCIe 3.0 so they should be fine with two cards;
- SB-e and IB-e should be fine with up to four cards;
- Nehalem should also be fine with two cards since it has 36 PCIe 2.0 lanes;
- AMD's FX line has 32 PCIe 2.0 lanes if you go for the 990FX chipset, so it's fine for two cards, but going for 3 or four cards drops it to 8x PCIe 2.0 bandwidth per card, AMD's 990X can only do two cards at x8 each so that's a good question.
- Probably the most important platform where many people are still satisfied with the performance and don't have a reason to move is Intel's SB, which only has 16 PCIe 2.0 lanes, which puts it in the same situation as AMD's 990X platform.
All in all, having this in consideration, AMD must have gotten the bandwidth demands to not exceed PCIe 2.0 x8, along with the GPU-CPU communication that also has to take place.
Assuming it actually passes raw video data (I can't think of it doing anything else), everyone with Ivy, Haswell, and AM3(+) FX chipsets should be fine for at least tri-fire. Sandy should be good for Crossfire. SB-e and Ivy-e should both be good for quad.
So ya, your assumptions would be correct. Even Eyefinity 4k will not surpass the bandwidth of 2.0 x8 until you add a 4th card, although you wouldn't want to run a card that way anyway.