Nehalem cpus are largely insensitive to ram speeds, or single/dual/triple channel operation. As a rule more ram is better than faster ram.
RMA stands for Return Merchandise Authorization. It is the process by which you return a defective product for repair or replacement.
I suggest you properly check out your ram, and the ram slots.
Use memtest86+ You should get NO failures.
Read your X58 motherboard manual. There will be specific slots where the differing size ram sticks need to be installed to get dual or triple channel.
You may have caused an issue if the sticks are mismatched, particularly for voltage. If so, just install the 6gb kit in the correct slots, and you should get triple channel operation.
If you get a failure, test the sticks individually to find the good/defective ones.
Then test, using a known good stick in each of the slots to check for good/bad slots.
This may not be possible on your board, since early X58 motherboards were somewhat restrictive on which slots needed to be filled if you used less than the max.
Hi - Thanks for replying, will look more into it after work!
My issue with the motherboard is that it no longer reads the correct amount of memory and uses single channel instead of triple.
I have been using it for over a year, I was using 6GB installed to 2,4,6 channels I think they are called? Anyway, it was no problems - everything was good. I was able to change the speed in bios etc (nothing fancy, just the premade options)
But after my removals, I noticed it was only reading 2GB. I tried to re-seat, but same issue. So I bought a 12gb kit and that was only reading 4GB. So I added the rest, which totals it to 18GB - from that, it was reading 6GB!
When I open CPU-Z it detects I have 18GB installed, when I open like CCleaner or during bootup, it detects 6GB. And CPU-Z says its on single channel.
I'm glad to hear that RAM isn't causing as big of an issue as I thought.
You have a faulty mobo -- you can follow the geofelt (post above) advises to find out which slots are working and those that don´t. However if it is still under warranty, then the best thing to do is to open a RMA.