Unbelievably easy.
According to
Dell's document, the Diagnostics Indicators code (only 1 & 3 are on) means "Memory configuration error". Then I just unplugged two 512MB memory modules from the main board (of course, disconnect the power cable first), and then power on. POST succeed and it showed message on the screen saying that the amount of memory was changed:
The amount of system memory has changed.
Strike the F1 key to continue, F2 to run the setup utility
Then press F2 to view CMOS settings. No problem was found.
After doing some testing, I added the two modules back (and the above message showed up again). Reboot, and it worked. Done.