I have continued to explore the possibility of 2x RTX 3080.
2x 3080 air cooled need at least 1 slot free above and below each card.
All well air cooled, dust protected cases are only suitable for 2x 3080 (except maybe Founders Edition) put into the PCIe slots directly. Riser cables to manually built basements, whose fixation is unclear, for the cards are in conflict with case walls.
2x Nvidia RTX 3080 Founders Edition has 2 slots width. Therefore, they fit into mainboards having at least 2 PCIe "x16" slots each with 4.0 x8 (or 3.0 x16, which is typical for Intel-CPU mainboards) speed in dual use and that are 3 slots apart. Such X570, LGA1200 or LGA2066 mainboards are available after the most meticulously careful study of specifications and handbooks (compare my earlier warning) for ca. €270+. (For comparison, a simple B550 mainboard is available for ca. €100 and works for 1 card.) If you can accept the Founders Edition, such a solution works. I consider this card somewhat too loud.
I guess that 2x 3080 water cooled costs at least ca. +€500 ~ €600 (of that >€300 for the cards' cooling in case of self-assembly). If you consider 1x RTX 3090 good enough (although it is only 15 - 20% faster than the 3080), you hardly save money because the card costs €1500.
How about 2x 3080 each with almost 3 slots width? We need a mainboard having at least 2 PCIe "x16" slots each with 4.0 x8 (or 3.0 x16) speed in dual use and that are --- 4 --- slots apart. These current suitable mainboards exist:
Gigabyte X299X AORUS MASTER (rev. 1.0), €380, socket 2066, 48(?) lanes (*1).
MSI Creator-X299, €450, socket 2066, 48 lanes.
MSI MEG X570 Godlike, €580, chipset X570.
Some Supermicro mainboards (*2).
(*1) Beware the X299X spelling! (X299 is a different mainboard.) It is unclear if 2 cards do run each @ x16 speed without activating SLI. The manuals have been written by copy&paste. One should ask the manufacturer if 2 non-SLI cards do run at that speed.
(*2) C9Z490-PG, C9Z490-PGW (chipset Z490); X11SAT (chipset C236). The manuals are hopelessly ambiguous concerning the PCIe slots in which to put 1, 2 or 3 x16 cards. Therefore, I cannot confirm whether a distance of at least 4 slots can be established. Ask the manufacturer.
Intel i9-10900X is a typical LGA2066 / X299 CPU at €530. For AMD AM4 / X570, we can assume a comparable Ryzen 9 3900X (2 cores more, slower clock) at €400.
Hence, let me estimate the excess expense in € for 2 instead 1 RTX 3080 on a cheap AMD B550 mainboard:
Code:
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AMD Intel Item Details
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>+700 2nd RTX 3080
<+480 +410 mainboard/CPU AMD +0 CPU. Intel +280 mainboard +130 CPU.
+140 PSU 1500W (Be Quiet) instead of 1200W (Corsair).
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+1320 +1250 sum
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47% 44% fraction of expense for non-GPU items
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IMO, the extra cost for the mainboard and (in the case of Intel) CPU is too large to justify building a PC with 2x 3080 having a 3-slot design. Financially, the Founders Edition is the only viable option for 2x 3080 and, on an AMD mainboard, maybe even a quality 1200W PSU might be enough. (I can't guarantee this.) For an extra expense of +€700 or 840, one might build such a PC.
I won't because I prefer a quieter and cooler RTX 3080. Saving some money now gives me an easier option to upgrade when Nvidia can use 5nm, 3nm or 2nm in a few years.
Is my analysis reasonable or do you have alternative suggestions of how to build a 2x 3080 3-slot-design PC without spending too much on mainboard and CPU? Note that I have already disregarded an open or bench case build for myself.
EDIT: If you want to build 4x 3080 water cooling this mainboard (with 2 slot distance between x16 speed slots) is available: Gigabyte X299-WU8 (rev. 1.0).
EDIT 2: The problem with current Intel CPUs and chipsets is, besides price, that they are not future-proof without PCIe 4.0. If one wants to build high end Intel, it is better to wait for new Intel CPUs and mainboards with this feature. Although it can be ignored at the moment, spending thousands on a new gaming PC should last for several years. This is uncertain with PCIe 3.0. Only for an industry / business PC, one need not care about much else than stability.