I now had 2 RAM modules
- A 512 MB 400 MHz module from Simmtronics.
- A 256 MB 333 MHz module from a lesser-known brand.
As I was about to remove the 256 MB module, an idea flashed in my mind. I quickly swapped the positions of the modules and powered on my system once again, with higher hope. And guess what? It booted normally! The BIOS showed my system's memory speed as 333 MHz, and the total system memory as 768 MB.
I'm not sure why interchanging positions of the two RAM modules made things work, but what I suspect is this (and this is the idea that had flashed in my mind). Putting the 333 MHz RAM into the first slot caused its frequency to be set as the system's memory frequency, by the BIOS (during power on). And this frequency was imposed upon the module in second slot. Since in this case it was a module that supported higher frequency (400 MHz), it simply ran at a slower frequency, without any problems.
However, if the 400 MHz module is put in the first slot, 400 MHz is set as the system's memory frequency (by the BIOS), and the module in second slot isn't able to support it, triggering the error beep sequence.
I again emphasise that I'm not sure about what in reality is the reason for what happened, and that this is the idea that came in my mind. Most importantly, it worked!