Thanks for the replies.
I think I solved the mystery.
When I installed the new module
, it forced the mobo to adjust the memory voltage to 2.0v to accommodate the module requirement. This overall increase in motherboard voltage probably freaked out the old module, which is rated 1.8v
Since this mobo voltage increase is automatic and not easily reversible, the computer failed to boot even after I removed the new module and left the old module in its old bank.
When I inserted the old module back (along with the new module), the computer failed to boot again.
To solve this problem, I just ordered another module to match the new module. As nyterage recommended, having two identical modules would circumvent the mismatch issue and allow a real upgrade.