I've found this rather interesting hardware. Backplanes for single board computers (SBCs) that have tons of PCI-e slots. They have various levels of bandwidth (4x, 1x, 8x, 16x, and mixes of these, some gen 2 and some gen 3). What are the PCI-e bandwidth needs of GPUgrid.net (and other GPU projects)?

here's the backplanes I speak about: http://www.trentonsystems.com/backplanes/
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Ronny depends on what cards you want to use. For fermi still pcie1 x4 like in my case is enough. About kepler,maxwell i dont know but im sure other know ;) for einstein you need x16 gen1 as minimum for performance on fermi, im sure much more is here needed to on kepler and maxwell. So you must ask on eatch project website what are there requierments.
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DSKAG Austria Research Team: http://www.research.dskag.at