Have you called EVGA's 24/7 Tech support at 1-888-880-EVGA, as you will need a service ticket # prior to an RMA anyway, and they may have additional things for you to try.

I would suggest Memtest 86+ but you said you were perty sure the ram was good, which leaves the board, or the cpu, or your settings, Your running XMP at stock which is supposed to set your settings, I assume you have a 64 bit Op system, as you cant see all your ram with a 32 bit system, and Other than the low gap Bios setting, or trying standard memory setting with a 2:12, +200 Vtt and a 24X unicore, with DimV at 1.65, for 1600, or a 2:10 for about 1333, or a 2:8 for 1060; I'm not sure what else you can try. ( Note unicore freq should be 2X your multaplier, so 16X @ 2:8, 20X @ 2:10, and 24X @ 2:12.

Try setting the timings manually. For example:

Memory Feature   
Memory Speed ( Standard ) 
Memory Control Setting ( Enabled ) 
Memory Frequency ( 2:10 ) 
Channel Interleave Setting ( 6 Way ) 
Rank Interleave Setting ( 4 Way ) 
Memory Low Gap ( Auto ) 
tCL Setting ( 8 ) 
tRCD Setting ( 8 ) 
tRP Setting ( 8 ) 
tRAS Setting ( 24 ) 
tRFC Setting ( 74 ) 
Command Rate ( 1T ) 


If with 1.65 vDIMM and factory timings, the system still only shows 4GB of memory, this can possibly be the memory kit. I ended up having to RMA a memory kit recently due to memory dropping due to running factory timings. It could also be a bad slot. You could try and run one stick by itself in each of the three slots and see if the system posts in each case.

Just got home from work and booted up. Voltage changes made no difference as I booted up with 4gb again. Loaded bios defaults and its back to 6gb.
UPDATE: Well I just tested all the sticks indepently and they all booted and passed the windows memtest, I installed them in a different order as well. I also reseated the cpu cooler as I have read that can be an issue, I made sure all 4 screws were equally tightened and then powered it back up. I see 6gb now but I'm guessing that when I boot up tomorrow morning I will be back to 4gb and will have to load defaults in bios, reboot back into bios, load saved profile, and then boot back into windows to get all 6gb back.
 
I'm thinking about RMA'ing and stepping up to the classified now... even though I don't want to do that,  but if I have send it back I might as well step up. Do thet classifieds have memory issues as well?

I have now re-seated the cpu as well and I am still having the same issue. After all I have done the issue remains un-changed. I will be submitting an RMA request and stepping up to the e760 Classy board as well. Hopefully this will resolve my issues, I would like to start enjoying my system...

sorry I don't have any useful solutions, but I just wanted to throw my voice in the mix. My x58 SLI LE suffers from the same problem. Since purchasing it, I've returned and replaced it once already, both memory and motherboard. The Corsair memory I have definitely isn't the issue. It seems to be the motherboard. At stock, I usually have to start the system up, turn it off, and then turn it back on again in order to get all 6 gigs to be recognized. I've tried updated the BIOS to the latest version and of course memtest86+ showed that all sticks were normal. At least I'm able to get to the point where the system will recognize the 6 gigs. *sigh*

I had to go in the BIOS and reset to default and reboot then I needed to adjust the timing manually, setting the ram to XMS profile just caused issues. I  have a i7 930, EVGA X58 SLI(E758) and 8 Gig of Corsair Dominator GT 1866. Make sure the voltage is 1.65 for the DIMM and the vtt you may want to put it 150-200, also put the timing to 9-9-9-24, I also needed to put the timing on of of them that is usally 85 but by putting it to 120 help, just don't recall the last one.

Yeah, thats the conclusion I have come to. To me, its seems to be an issue with the board posting and not initializing the ram correctly. When you make and save changes to the bios I believe the board posts differently and thats why it then sees all the ram. If I just put it to sleep is S3(suspend to ram) it then does not need to post as the ram is staying active, it works great. Anyway, I dont care anymore, sick of troubleshooting with no results. Shipping out my 758 tomorrow for step-up. Can't wait to get my Classy!

This problem is, for some reason I can't fathom, a "common" problem with P55 and X58 boards. If you check different forums, you'll see several people have had this exact same issue with various brands of motherboards.