There's no two ways about it, if you want to build a Coffee Lake system, you need a new Z370-chipset motherboard to go along with the processor—you can't run a Coffee Lake chip in a Z270 motherboard. That said, you can take some of the sting out of upgrading by shopping Newegg's bundle deals. One of those deals is for a Core i7-8700K processor (opens in new tab) packaged with MSI's Z370 Gaming Pro Carbon AC motherboard (opens in new tab).
Purchased separated, the two would collectively set you back $590, or $570 after cashing in a $20 mail-in-rebate on the motherboard. Newegg's bundle deal drops the price to $550 upfront, and $530 after factoring in the rebate.
The Core i7-8700K is a 6-core/12-thread processor clocked at 3.7GHz (base) to 4.7GHz (Turbo), with 12MB of L3 cache. It has an unlocked multiplier, though whether you plan to OC or not, you'll need to bring your own cooler—it doesn't come with its own. We recommend a decent liquid-cooling solution (opens in new tab) as a minimum if you plan to overclock. So basically put the savings from the combo into a cooler.
MSI's Z370 Gaming Pro Carbon AC offers a decent assortment of features, including three PCEe 3.0 x16 slots (two are reinforced), three PCIe 3.0 x1 slots, six SATA 6Gbps ports, two M.2 slots, USB 3.1 Gen2 connectivity (Type-A and Type-C), and of course RGB lighting. It also comes with an Intel dual-band Wi-Fi card.
You can find the bundle here (opens in new tab).
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