How to Perform a BIOS or CMOS Reset and Clear the NVRAM on Dell Systems 

1. Before you begin, follow the Safety Precautions When Working with Electronic and Electrical Equipment.
2. Turn off the computer and disconnect the power cable from the computer.
3. Remove the computer cover. For model-specific information, see the User Manual of the Dell desktop.
4. Locate the 2-pin password jumper labeled PSWD on the system board.
5. Remove the 2-pin jumper plug.
6. Locate the 2-pin CMOS jumper labeled RTCRST on the system board.
7. Move the 2-pin jumper plug from the password jumper to the pins on the CMOS jumper.
8. Plug the power cable to the computer and wait for 10 seconds for the CMOS to clear.
9. Disconnect the power cable from the computer
10. Move the 2-pin jumper plug back to the password jumper.
11. Replace the computer cover. For model-specific information, see the User Manual of the Dell desktop.
12. Connect the power cable to the computer and turn on the computer.

Figure 1: Illustration of the jumpers on the system board of a Dell desktop computer.

By the way, I don't think they worded that quite right.  Looks more like BIOS password reset.  What you'll need to do is remove CMOS jumper.  Plug in PC for 10 sec.  Unplug and then put CMOS jumper back.

Rare I suggest it this way as replacing battery alone should reset CMOS/BIOS.  The beep code the 790 is doing doesn't match the codes in the owner's manual.

While it could be a sign of trouble, like MB trouble, if everything else is working right, I'd be tempted to disconnect the beeper.

You also already did a good job of troubleshooting.