Specs:
Windows 7 Ultimate x64
Asus P6X58D
Intel Core i7-930
3x 2GB Corsair DDR3 1600
2TB Seagate Barracuda XT
300GB VeolciRaptor
PNY Nvidia Geforce GTX 260

The 6 GB of RAM is recognized in POST, but not in Windows. All the RAM tests okay. I have booted the computer with each stick in the motherboard and each one is recognized on its own. I also tried each DIMM and RAM is recognized in each of them.

I have also flashed the latest BIOS. Any suggestions?

How much is recognized?

Only 4GB is recognized.

This may be a stupid Q but are you sure its 64-bit OS?

Is it recognzing exactly 4.0GB or 4 and some change/ 3 and some change?

The only thing I can think of is 2GB is being allocated to video.

100% certain that it is 64-bit.

In the system properties it shows 4.00 GB. In the BIOS it shows it spelled out in MB as 6144MB, if I recall correctly. It appears that sometimes when I re-arrange the memory and reboot, the full 6GB is recognized. But I have to restart a few times to get it to recognize.

The only two things I can think of is either it's allocating 2GB to your graphics card which is unlikely because it's got built in
900mb memory. While the system still may share memory with the card even though it's got almost 1GB, sharing 2GB is highly doubtful.

My other thought is a bad memory slot. I know you said you tested the sticks individually, did you test the slots individually?

Yeah, all the DIMMs tested correctly. I toyed around with some more advanced settings in the BIOS and it seems to have settled itself after I modified the profile for the memory. It was set to auto and I adjusted it to XMP and manually set the timings for the RAM. After some quick research I seem to have confirmed this. It appears since the memory processing is now in the CPU that voltage is quite a bit more touchy now with RAM. Haha.

Okay, so it looks like the problem has returned. I have set all the timings manually, reseated the CPU, and ran MemTest on the RAM. I ran it for 8 passes and it found no errors.

This is an intermittent issue. Sometimes it will go several boots recognizing the memory, but then all of a sudden it will drop down to 4GB (which upon double-checking, it shows up this way in the BIOS too). If I re-seat the RAM, it usually recognizes the RAM again. Each stick is recognized in each DIMM. I don't want to have to try running single sticks at a time for several boots in order to see if it is a particular stick or not.

To add: I will be booting with each stick to see if I can find a specific one, and RMA it if I find it.

Hm, this is quite perplexing. It seems to happen much less frequently now that I have enabled XMP profile with the proper timings manually entered. It is on average 1/10 boots. All the sticks pass tests. I'm still playing with it, but any suggestions would be appreciated.