The Core i7-7800X 6-core/12-thread chip ships with clock speeds of 3.50 GHz, and 4.00 GHz Turbo Boost; and 8.25 MB of L3 cache. You get a full quad-channel DDR4 memory controller; but like the "Kaby Lake-X" chips, this chip too has a limited PCI-Express budget of 28 lanes, so you can't run two graphics cards at full x16 bandwidth (wasn't that the whole point of the HEDT platform?). The i7-7800X is priced at $389, just $50 more than the i7-7740X, which seems like a bargain for the two extra cores and a whopping 0.25 MB of more L3 cache.
The Core i7-7820X 8-core/16-thread part is clocked at 3.60 GHz with 4.30 GHz Turbo Boost; featuring 11 MB of L3 cache, a quad-channel DDR4 memory controller; yet the same limited 28-lane PCIe root complex. It is priced at $599. Back in my day, a $279 Core i7-920 paired with any X58 motherboard to give you full x16 lanes to two graphics cards, and enabled 3-way and 4-way multi-GPU configurations. If that's what you want, then get ready to pay top-dollar for the Core i9-7900X.
The Core i9-7900X at $999 is your price of admission for the 44-lane PCIe root complex of the "Skylake-X" silicon. This 10-core/20-thread processor is clocked at 3.30 GHz, with 4.30 GHz Turbo Boost, and 13.75 MB of L3 cache. Both the i9-7900X and i7-7820X feature Intel's new Turbo Boost Max 3.0 feature, which adds a further 200 MHz to the max Turbo Boost frequency, if your cooling is satisfactory. All chips being launched today feature unlocked base-clock multipliers, which make overclocking a breeze. All "Skylake-X" chips feature TDP ratings of 140W.
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