with some help I bet you could hunt the components on sale and get both the rx570 and a better CPU...but if you are not that familiar with this and you are more comfortable getting a prebuilt PC that you know will work well I completely understand.Just saying, buying the parts and then building it can be friendlier with your pocket.
This is a good point. OP, what is the price you're looking at with this? I've been helping people get the absolute best budget penny pincher gaming rigs for ages now, and find that especially now, the best value is to pair a used Xeon E5 workstation with a mid-range GPU. Done right, this takes care of a ton of stuff in one fell swoop, and is easy to do.
Start with a Lenovo S30, Dell T3610, or HP Z420 type system. They all have excellent cases built very well, with easy access. They almost invariably come with either a Win7 Pro COA or UEFI OS license, either of which will activate full legal Windows 10 Pro, another savings. They all have 8 Ram slots and use very affordable DDR3 ECC memory in quad channel 1600/1866 speeds, which is effectively equal to DDR 4 3200/3466 bandwidth, only with a bit less latency. The S30 and 1610 in particular have excellent stock PSUs capable of running decent GPUs right out of the box either with 6+6 PCIe power or 6+6=8 pin adapter for slightly hungrier GPU types. 610W and up gold rated PSUs meant for 24/7/365 business workstation use, good stuff. And then, plenty of expansion capacity with PCIe slots and channels (40 lanes of 16x 3.0 speed!).
As an example, here is one that had an E5-1620 Xeon (4C/8T 3.6Ghz, 3.9Ghz Turbo, 12MB Cache), 64GB (!!!) Quad Channel Ram, etc for under $150 sold. Literally add a cheap boot SSD and your choice of budget GPU and you have something fully capable of great gaming for an incredibly good value. They usually come with some kind of older Nvidia Quadro GPU which isn't good for anything like AAA gaming, but is often enough for Fortnite/Minecraft/Indie gaming if you are just setting up a kid with their first PC of their own, which can be upgraded later. Keeping the Quadro as a spare is nice in case you want to sell your fancier GPU while waiting for new models to ship, or having to get that money in your pocket before upgrading to a new one, gives you the way to keep your PC up and running in between better cards. You can also add stuff like PCIe USB 3.1/C or 10Gbe network cards for affordable feature upgrades if you need anything like that.