The X299 chipset is Intel's latest HEDT (High-End DeskTops) solution. It sports the new LGA 2066 socket, which supports Skylake-X and Kaby Lake-X processors, with between four and 18 Hyper-Threaded cores. The X299 chipset features 24 PCIe 3.0 lanes and 10 USB 3.0 ports, which is a notable improvement over the outgoing X99 chipset's eight PCIe 2.0 lanes and six USB 3.0 ports. SATA III support is somewhat reduced on the X299 chipset when compared to X99, but this is offset by the more important inclusion of Intel Optane support. Intel also implemented a DMI 3.0 connection between the CPU and chipset, which provides twice as much bandwidth as the DMI 2.0 interface used on X99.
Intel's X299 trades blows with AMD's competing X399 chipset. AMD gains an advantage here by incorporating native support for two USB 3.1 Gen 2 ports, but it suffers from its more limited PCIe controller, with only two PCIe 3.0 and eight PCIe 2.0 lanes. Threadripper's plethora of 64 PCIe lanes helps to offset this, as the CPU itself far surpasses Skylake-X's and Kabylake-X's maximum of 44 PCIe lanes.
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Intel X299 Motherboards
11/29/2017 Update: added the ASRock X299E-ITX/ac (earned a 2017 Editors' Choice award).
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Selecting an X299 motherboard will likely take more thought than you'd put into a lower-end board. Because of their rich featureset, these boards are often more expensive. Plus, you'll likely have a specific need for all that horsepower. Perhaps you are interested in the system purely for gaming or compute purposes, in which case you'll want to pay close attention to the spacing of the PCIe x16 slots to ensure you can fit two, three, or even four GPUs in. Alternatively, if you need lots of fast storage, you'll want a board with plenty of M.2 Key M slots and SATA III ports. Most X299 motherboards will have ample amounts of both, but the point remains: you want to shop for your specific need above all others.
In general, there are only two factors that everyone should pay attention to on X299 motherboards. The first is that the PCIe layout matches the specific processor you've chosen, since Kaby Lake-X processors only have 16 integrated pathways while Skylake-X has either 28 or 44 (depending on the model). Some motherboards achieve greater drive bandwidth by using CPU rather than chipset lanes for M.2 slots, and combining the wrong processor with the wrong motherboard layout can result in some of your required interfaces being disabled. Another major feature of the X299 platform is its support for quad-channel memory, which allows Skylake-X processors to support a total of eight possible DIMMs. Meanwhile, Kaby Lake-X only supports four, so motherboards that have four slots leave Kaby Lake-X owners with only two functioning. Even if you won't use all slots now, an eight-slot motherboard opens the door for potential upgrades down the road.
The other key area you want to focus on is the power delivery system. This gets a bit tricky with high-end motherboards of this caliber, as the additional four DIMM slots typically invade the area traditionally reserved for power regulation hardware. Nonetheless, the last thing you want is your new PC to throttle due to an insufficient number of phases or inadequate cooling. Ensure the motherboard you pick has plenty of both.
EATX Intel X299 Motherboards
ATX Intel X299 Motherboards
Mini-ITX Intel X299 Motherboards