Mixing ram can cause problems, if I were you I would get all new memory. That being said, you can still actually buy a triple channel Dominator in 12gig. Also, if you ever overclock having all six slots full as oppose to only three puts more stress on the memory controller which normally makes you need more vtt.
 
Linky to the egg

soon, i have to offer a 6 x 2gig dominator kit...can you post a picture of yours, so i check mine, maybe its the same type and you can get 3 x 2gig for a good price...

if I remember correctly the maximum cpu fsb on the x58 was 1066. your 1866mhz is over kill and you already have unused bandwidth. I used the 990x cpu and any memory over 1600mhz actually was less stable and hindered the performance more than improved it. your 920 cpu cant keep up with your memory as it is, so unless your memory is bad I wouldn't  worry bout your memory. adding more memory probably wont help. maybe consider a faster cpu.

VIPERIXIX
if I remember correctly the maximum cpu fsb on the x58 was 1066. your 1866mhz is over kill and you already have unused bandwidth. I used the 990x cpu and any memory over 1600mhz actually was less stable and hindered the performance more than improved it. your 920 cpu cant keep up with your memory as it is, so unless your memory is bad I wouldn't  worry bout your memory. adding more memory probably wont help. maybe consider a faster cpu.

This isn't entirely correct, it does depend somewhat on who made the motherboard but I know my x58 evga lets me use up to a x12 memory multiplier and can get you near 1600mhz with stock base clocks. As far as memory being less stable at over 1600mhz, either your memory not capable of it or you are not overclocking the cpu correctly. I know the reason I just got more memory was because 6gig was no longer enough for me. I was finding myself sitting at 5.7gig or more used memory more and more often which did impair performance because it began using more paging files and accessing the hard drive more often. All that being said, above ~1600mhz the performance boost due to the higher frequency isn't really noticeable unless you do a lot of heavy computing. I have a 2000mhz kit just to give me room to play when overclocking, normally its running in the 1500-1700mhz range with tighter timings. 

VIPERIXIX
you must have misread my post....the memory bus on the 920 cpu is the issue.....my 4 way sli board bios clocks up to 4000 MHz and I have run2400mhz on that board stable but the memory clock maxes out at 1866 on the cpu memory bus on the board. the max on the oc. on the cpu memory bus past 1600mhz is where it becomes unstable. and your qpi is 4.8 and what I was trying to suggest to you was the higher cpu will increase your performance and stability with a higher qpi and an unlocked cpu and higher bandwidth 10 fold compared to adding memory past 6 gb. and filling all 6 dims also reduces oc stability. If you been able to clock that 920 memory bus at 1866 please share those settings.
I just wanted to clarify.

It is possible to get over 1866 for your memory speed, as that is a high speed silicon lottery will affect if the voltage needed to get there will be too high for 24/7 use. The memory speed is directly linked the base clock though a multiplier that can be set up to 12 on most evga boards. So even a modest overclock of 3.4ghz on the cpu using 165 as the base clock will give you the option to go up to 1980mhz on memory because 165 with the multiplier of 12 is 1980mhz. Now that being said to get that stable you may need something like +150mv or more vtt. But its possible. Also simple because a processor is unlock does not mean that it will always overclock better. Locked xeon x5650 tend to get the same overclocks as the unlocked 980x and use less voltage in doing so(on water and air). Unlocked cpu's are simpler to overclock because you don't have to worry about memory speeds changing with your base clock. I do agree having 6 dimms will make it much harder if not impossible to run above ~1600mhz because of the extra load. Which is why I suggested a 3*4gig kit. Also no idea why having a "higher cpu" would be more stable. Stability can be achieved in any case as long as you overclock properly. 
 
There is more then one reason why more memory can help out. If he does video editing the extra memory will help out, if the computer has to keep accessing the hard drive because its memory is full it will not matter how fast his cpu is the hard drive will just bottleneck everything.