In other news, I just confirmed a suspicion that not all the slots may be useable. Fortunately, only one of the 18 slots won't work, not 2 like I had previously thought. This is because the slot nearest to the SHB doesn't actually connect to the 2 main fanout switches on the board - instead, it routes to the x4 connector that is adjacent to the host board, and that x4 connector is used for an optional extra daughtercard that attaches via another connector to the SHB. Even if I had that expansion card and could plug it into the current board, I'm not sure that the new board (arriving Tuesday) will have that expansion capability. So as of right now, 17 useable slots.

Additionally, I've been thinking harder about a rack formfactor, and how to achieve it. Basically, the 2 main constraints are a maximum case width of about 17 to 17.5 inches, and a maximum depth of about 27 to 28 inches. Vertical space in a "normal" system that uses this board is 4U (7 inches), but this system isn't normal.

If I aircool, I need 2 levels of GPUs, so rack height without PSUs is 6-7U. PSUs might be able to fit into the depth (they are about 12 inches deep), but the fans in them blow the wrong way, and the power cords would then be coming out the "front" of the rig, from a rack perspective, which is awkward. If standing up, they need 3U, but turning them on their sides buys me 1U of height.

If I watercool, then I need the depth for manifolds, hose, and other apparatus, so the PSUs would still have to stack, but all of it could probably be crammed into 6 or 7 total U. As much as I would love to have a package system, I doubt I could fit an appropriate radiator and pump into that space, so it would probably have to be external like DeathAndTaxes' setup. BTW, has anyone actually done any long term tests with more than 2 watercooled cards directly adjacent to each other without a second slot in between? The more I consider it, the more I am sure it would be prone to failure, unless they weren't linked directly to each other. If that were the case, I would have to use right angle connectors on each and every card, and that would get tedious, real fast.