I also have an ASUS Motherboard. P5QL/EPU. which takes up to 16 GB memory.
In the USER GUIDE that came with the Motherboard, it says :-
Due to the memory address limitations on 32 - bit WINDOWS OS, when you install 4GB or more memory on the Motherboard, the actual usable memory for the OS can be about 3 GB or less.
For effective use of memory, we recommend that you install a 64 - bit WINDOWS OS when having 4 GB or more memory installed on the MOTHERBOARD.