After trying the following: (1) tweaking with maximum memory settings in msconfig (2) flashing bios to the latest version without finding a "memory remapping" option, I tried the 3rd most feasible method: reseating the memory, however this is when more problems suddenly showed up.
This was working before reseating (showing 12GB installed, though only 5.96 GB usable), but not anymore. It will give 4-4-4-4.. beeping codes.
Now the configuration is
Slot 1 4GB
Slot 2 4GB
Slot 3&4 empty
But windows 7 only tells me 4GB installed. It seems not to detect the other module in slot 2 at all.
I just verified that all modules (2 4GB modules and 2 2GB modules) work when each of them is seated in slot 1 alone.
It seems the computer now only recognizes module in slot 1 and does not even allow me to put anything into slot 3 or 4 or it will beep. I feel it unlikely that I broke 3 memory slots, because slot 1 is still working fine despite the most attempts to insert into it. Also I grounded myself before putting them in and upgrading memories has never been a problem for me.
Btw, the two 4GB modules I have are 1600MHz but since they were recognized in the first attempt, I assumed it was not a problem.
Thank you for pointing that out Blazorthon!
The current problem, however, is that with 8 Gigs (2x4GB matching modules) installed, only one of them is detected after normal booting.
Bios, windows 7 and crucial memory testing application all report just 1 module of 4GB installed. The presence of the other module is not reported anywhere.
But if I replace the undetected module with a non-matching 2GB module, there's gonna be beeping sounds without starting properly.
It's already clear that both 4GB modules are usable. So I'm not sure if I broken the memory slot or something's wrong with BIOS...