The BIOS setting disable CSM in default in order to install Windows 8. If you need to install Windows 7, please enter BIOS setting and go to [Boot] tab. Click [CSM parameters] and set [Launch CSM] to [Enable]. Save and exit, enter BIOS setting again after reboot, then you can set up [Hard Drive BBS Priorities] under [Boot] tab.
The 20-pin connector belongs to older ATX standard while 24-pin connector follows newer ATX standard. The 24-pin connector just the 20-pin cable with 4 extra wires to provide extra current. As long as your power supply can provide sufficient power to the motherboard, you can still use 20-pin power supply.