There’s just the one HD. Has space for 6, full size!
Definitely will try one RAM stick at a time. That’s easy peasy.
If you have a multi-meter you could measure all of the power supply voltages at the connector.
“All PC power supplies provide at least +5VDC, +12VDC, and -12VDC. Some power supplies also provide +3.3VDC, -5VDC, and/or 5VSB.
Remember that the wire color represents the voltage. Here is pic that shows the color and voltages on a common power supply connector.
https://i.sstatic.net/XfJWS.jpg
