No Dual Channel

Well first you've been lucky running you Phenom II X4 965 @ DDR3-1600 without FSB -> 240MHz or CPU-NB Frequency -> 2400MHz.

Sometimes ASUS screws-up reading 'SPD' when set to Auto. Trying to hope it's a matter of BIOS if you post a link to your "2X 2GB GSkill 1866" or better a screen shot of the 'SPD' tab or JEDEC timing/volatge data from the about (TXT)-- I'll post some choices and manual settings to try.

Running DDR3-1866 on a 4-core Phenom II X4 is dicey at best, so I'll post for DDR3-1333 and DDR3-1600 as proper configurations.

it doesnt work because your putting the sticks in the wrong slots. use the first and third or the second and forth. the same color slots wont work.
asus does this alot

I think the OP is using the correct DIMM slots "two sticks of RAM in the blue or the black slots", if the OP said AND vs (or) then that would be another issue as you stated.

i read the manual. he is using the wrong ones.

edit: added plural

The OP has one set 2x2GB, either the Blue (preferred) or Black slots work as Dual Channel:

CPU: | blank | blank | 2GB | 2GB | ; or
CPU: | 2GB | 2GB | blank | blank |

if 2 kits for Dual Channel:

CPU: | 2GB | 2GB | 2GB | 2GB | ; 8GB
CPU: | 4GB | 4GB | 2GB | 2GB | ; 12GB
CPU: | 4GB | 4GB | 4GB | 4GB | ; 16GB

CPU: | A2 | B2 | A1 | B1 |

I don't know what you're reading?!

---

You suggested:
CPU: | blank | 2GB | blank | 2GB | - this is wrong and not Dual Channel - I wasted my time looking at the manual.

Channel 0 is A and Channel 1 is A, in order to have 'Dual Channel' equal sizes per Channel 0 & 1 must be filled with any of the above (5) ways I posted.

i got the right manual here is an image from the pdf.
http://img854.imageshack.us/img854/8761/capturenfn.png

Yeah, one stick needs to be in the (A) and one in (B); Dual Channel = A1+B1, or A2+B2, or (A1+A2) + (A2+B2). Both Channels must be equal in size.

What I post above is indeed 100% correct, I have ZERO doubts.

mem ok light stay on in user manual page 1-28 means memory is not properly installed ,so use user manual they tell you how to resolve this

there is another thread here on tomshardware that covers this problem.
http://www.tomshardware.com/forum/297538-30-asus-m4a88t...
you have to set your timings manually.

Thanks for the input. The two blue slots are the preferred slots per the manual. My system used to run fine with them there @ 1600. Now it will only boot with one stick in the first black and first blue which is single channel.

They auto detected at 1600. It's also one of the choices in the BIOS. Be that as it may, I've tried everything, including changing to 1333 in the BIOS before moving back to the blue slots.

Under CPU-z what are the CAS timings and voltage for 667MHz and 800MHz? If you post the numbers, lets try to set the RAM manually.

The only way for the RAM to run @ DDR3-1600 is to either set them manually in the BIOS or to be running some BIOS invasive application e.g. AMD OverDrive Utility, or ASUS Ai Suite, etc ... uninstall them for now and IMO permanently; they cause more issues then they solve and OC'ing is dirt simple.