Make sure your RAM sticks are on the same channel.
The Dimm slots should be color coded.
With 4 of the same sticks it should not matter.
Also, check and see if you have Dual Channel enabled.
Pull the RAM and make sure it is seated right.
Also, clean the contacts and slot.
Make sure you have 64-Bit installed.
http://pcsupport.about.com/od/windows7/ht/windows-7-32-...
Run MemTest to check the RAM.
http://www.memtest.org/
Go into BIOS/CMOS and see if is sees all of your RAM.
Swap the RAM in the slots too.
Run 1x stick at a time and see if it works.
With the old RAM in make sure you have no OC.
Do a hard reset of the BIOS/CMOS.
Pull the battery and wait 10sec.
Make sure the RAM is compatible.
Here is the list;
http://dlcdnet.asus.com/pub/ASUS/mb/socket775/P5WD2%20P...