Gaming: 5800X3D Productivity: 5900x For GPU take the best bang for your bucks and area of application. Both are good GPU manufacturers More on reddit.com

Yes you can use both More on reddit.com

You can find the CPU support list here: https://rog.asus.com/uk/motherboards/rog-crosshair/rog-crosshair-viii-hero-model/helpdesk_cpu/ Along with the bios version that supports it. This is important because if the CPU you are intending to use is only supported by a newer Bios version, you usually need a CPU that works with the board in order to upgrade it. More on reddit.com

It depends on your BIOS version. This list ( https://rog.asus.com/ca-en/motherboards/rog-crosshair/rog-crosshair-vi-hero-model/helpdesk_cpu/ ) shows all of the CPUs you can put into that board along with which BIOS version youd need. From what I can see, if you have the latest BIOS, then practically any AM4 compatible CPU is able to be run in that board, from Athlon A6/A10/A12s all the way up to 5800X3Ds, 5950X and such. Now I dont know if I would recommend using something like a 3900X, 3950X, 5900X or 5950X in that board, as the first gen Ryzen mobos, even the top-tier ones in the X370 chipset, didnt have the greatest VRMs and they certainly arent as good as the ones on some of their later boards since even the highest-tier Ryzen 1XXX chips didnt take all that much power, while the bigger 3XXX and 5XXX chips can get pretty power hungry, especially when you start overclocking and such. But if you were wanting to run practically any of the 8-core chips you should be just fine, maybe try to setup your system so that the VRM gets some direct airflow from a fan or two to ensure that it gets some good cooling if you are planning on running any of the higher-tier 8-core CPUs, since as I said, the higher-tier 8-core 3XXX and 5XXX CPUs are quite a bit more power hungry compared to the 1XXX chips that your motherboard was designed for. More on reddit.com