Hi, From the board's specs:
4 x DIMM, max. 16GB, DDR3 1800 (O.C) / 1600 / 1333 / 1066 MHz, non-ECC, un-buffered memory
Dual channel memory architecture
* DDR3 1600 MHz or above DIMMs work only on the Orange slots for one DIMM per channel.
* When installing total memory of 4GB capacity or more, Windows® 32-bit operation system may only recognize less than 3GB.
And the P45 specs:
Memory Types DDR3-1066 / DDR3-800 / DDR2-800 / DDR2-667
# of Memory Channels 2
Physical Address Extensions 36-bit
ECC Memory Supported No
Although I couldn't find compatible 16GB kits listed on manufacturers configurators
http://www.crucial.com/store/listparts.aspx?model=P5Q3&Cat=RAM
or
http://www.gskill.com/configurator2.php?pid=2&model=982
you can try 4x4 GB 1333 RAMs. They might work. Look also on the Internet for 16GB cofigurations for your board.