The following things listed on this page are needed to build and use a gaming SBC.
Required Non-SBC Hardware
PC running an x86/x64 CPU OS to image SD cards or other storage options.
In theory, one could buy a Raspberry Pi 3B/3B+, a micro SD card containing NOOBS, and a Card Reader to image other micro SD cards or other storage devices with after installing Raspbian, and even use the Raspberry Pi as your gaming SBC afterwards. Please ask someone at the SBC Gaming Discord if they could do this before attempting it yourself as it is untested.
Card Reader that can read/write SDXC cards.
A USB card reader is generally recommended, though some PCs have a built-in card reader that will suffice in most situations.
If getting a USB card reader, and using a PC that has USB 3.0 ports, then also make sure the card reader is USB 3.0 for optimal imaging and backup speeds: USB 2.0 is fine if using a Raspberry Pi for imaging.
If already intending to use eMMC on something such as an ODROID-XU4, then you don't necessarily need this as you could buy an "eMMC Module Writer" instead, though you could use an "eMMC-to-Micro SD Adapter" with this. More info on eMMC available on Single Board Computers and Storage pages.
HDMI Cable and HDMI-compatible TV or monitor.
If using a monitor, you will need either built-in speakers, an audio output on the monitor for PC speakers to plug into, or an HDMI Audio Extractor for the best sound that the SBC can output on its own.
There are more options that allow for you to output to "RCA", "S-Video", "Component", "VGA", "DVI", and "SCART", though these are generally only available on the Raspberry Pi.
USB Keyboard - PC: Optional - Raspberry Pi: Required
If on a PC, you will not need this as you'll be able to remotely log into your device and use your PC's keyboard to enter commands or edit files. If using a Raspberry Pi to image a micro SD card or other storage device, then this is required.
USB Mouse - PC: Optional - Raspberry Pi: Required
If on a PC, you will not need this as you'll be able to remotely SSH into your device and use your PC's mouse/touchpad to manipulate files. If using a Raspberry Pi to image a micro SD card or other storage device, then this is recommend so that the more user-friendly Pixel desktop GUI can be used over the Terminal.