So, I'm currently using 3x1GB Balistix on an MSI P35Neo2-FR. However, I cannot get the 4th slot to work, regardless of what I do. I forget now that i'm at the office if the board would post or not when the 4th stick was inserted, but I pretty sure that it didn't. If it did, it would fail after a few minutes.
I've tried swapping all the sticks back and forth a few times and they all seem to work fine...
Looks like 3 alternatives: 1. Run 3 X 1 GB 2. Replace with 2 X 2 GB 3. Replace motherboard.
Some more options: 1: loosen the timings 2: up 1 notch on NB 3: up memory v a notch 4: lower ram speed 5: adjust ram skew