Is the PSU not adapted to the motherboard?
It looks like you lost 4+4 pin CPU cable somewhere. SS-650 comes with 2 cables: 8 pin CPU and 4+4 pin CPU. Please checkout PSU box.
Motherboard wants 8 pin from first CPU cable and 4 pin from second CPU cable (4 pin from second cable won't be connected).
Please do something like:
- Attach M/B cable to PSU (2 sockets) and than attach 24 pin cable to motherboard.
- Attach CPU cable 8 pin to PSU and than attach 8 pin to the top socket on the motherboard.
- Attach CPU cable 4+4 pin to PSU and than attach 4 pin to the remaining top socket on the motherboard. 4 pin won't be attached, it is ok.
- Attach PCIe cables to video card, etc.
Do I have to power the ATX sockets on the motherboard, and how?
Every powerful things on modern motherboard are powered from 12v top sockets that are connected to 12v PSU using 8 pin or 4+4 pin cables. Ancient motherboards in 2000 years had additional sockets to power PCIe slots. Modern motherboards are more simple.
Do not forget that you can't attach PCIe 6+2 pin cables to motherboard sockets. Otherwise motherboard will burn.
Do I simply have to left the PSU's CPU and PCI-E cables unused?
Yes, You can attach only one 8 pin CPU or only one 4+4 pin CPU cable. 8+4 pin power is required for top CPU overclocking only.
People can still make a great CPU overclocking with PSU that has only one 8 pin CPU cable. Use "PCIe 6+2 pin -> ATX 4+4 pin" adapter. Do not forget to check this adapter pinout with multimeter before attaching it to motherboard. Wrong adapters are very dangerous for motherboard.