I was trying to upgrade my parents' ancient emachine W3650 to 2 GB of memory.
I checked the emachine support site, which states that the mobo supports 2 GB of memory.
I bought a 1 GB PC5300 DDR2 memory module for the upgrade. After inserting the module into the second empty bank on the mobo, the computer let out a long single beep upon startup, then nothing. The system fan spins, but there is no signal to the monitor--not even the POST.
I took out the newly added memory, and turned on the computer again. To my dismay, the computer would not start anymore. Except this second time, there is no long beep with POST.
Now my computer is completely back to the original hardware configuration, but it will not boot up anymore.
double check the speed(mhz) on the RAM, sometimes when paired with an mismatching set it will cause a hardware error and wont boot, the smartest way to get around this is to always buy your RAM in sets of 2 that way incompatibility issues do not happen.
also double check to make sure the motherboard will support the 667mhz ram you are attempting to use.
if everything checks out, attempt using just the new stick alone in the computer, (usually the motherboard will automatically downclock the higher mhz ram to match the lower speed so that should not be a problem) if it does it is just incompatibility between the two sticks.
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.