MoBo: MSI760GM-P23 (FX) 760G AM3+ R (2 DIMM slots)
GPU:XFX 6770 Radeon HD
PSU: ROSEWILL|RD500-2SB 500W RT
MEM: 2Gx2|CORSAIR CMV4GX3M2A1333C9 R
HDD: 500G|WD SATAIII WD5000AAKX
CPU: AMD ATH II X3 455 3.3G AM3 RT
OS: Windows 7 Professional x64
I got it up and running but noticed in the BIOS that it stated that I only had 2GB of memory. Everything is up to date now (BIOS, windows, Drivers etc...) but it is still saying that there is only 2GB in the BIOS. Windows 7 Reports 4 GBs but only 2GBs are usable. CPUz also states 4 GBs and that I'm running single channel instead of dual.
So the first test i did was on the RAM. I removed both and then inserted just one stick into DIMM slot number one. I cut on the power and the PC would not post. I then inserted the second stick of ram into DIMM slot number one and again it would not post. I then inserted the ram into just DIMM slot number 2 and the Mobo successfully posted and launched windows. This worked with both sticks of ram.
I'm sort of new at building PCs, but I assume that this means that the Mobo's first DIMM slot is faulty? I did give a few other things a shot like increasing the voltage, resetting the CPU and cleaning out the DIMM slot with compressed air. I can send the Mobo in for a replacement, I just wanted to leave no stone left unturned (especially because sending in the Mobo means that I'll be waiting to play D3!!)
Most motherboards support most RAM of the same technology, even if it's not on the supported list. That list is just a list of memory kits that the manufacturer tested, not an exclusive support list. It basically says that "hey, these are what we've tested and shown to work, but other kits should work ,we just don't guarantee it".
The problem seems to be a faulty RAM slot, although as to what exactly is wrong with it, I don't know. It might have a bent or otherwise damaged pin (or multiple pins), it might be seated on the motherboard improperly, it might have a faulty trace on the board between it and the CPU, it could be one (or more) of even more possible problems. Regardless of the why, an RMA of your motherboard is the best course of action.