Asus or Gigabyte MB

[Shinma]

Presuming that you are referring to the GIGABYTE GA-M56S-S3 already mentioned,

If memory serves, *pun not intended*
The memory limitation/annoyance is caused by the nForce chipset.

If familiar/comfortable with the process,
Update the mobo BIOS to current,
Using only one memory module in the system.
And as usual,
Clear CMOS after BIOS flash
and then
Set requirements/preferences in CMOS/BIOS menu.
Remember to set memory module voltage... along with the other items,
Shut the system down,
Remove power cord from any power source,
Install remaining three memory modules,
Reconnect power cord to power source,
Boot system,
Double check/verify all CMOS/BIOS menu settings...

IF system boots and all is in order,
Feel free to rant and curse nVIDIA.
IF system fails to boot and/or recognize all memory modules,
Feel free to rant and curse at nVIDIA and GIGABYTE.