RAM compatability with motherboard - not POSTing in Dual Channel setup

RAM compatability with motherboard - not POSTing in Dual Channel setup

I have a problem in installing 2 DIMMs of G.SKill memory in my motherboard.
To start with
motherboard is http://www.msi.com/product/mb/KA790GX.html#/?div=Detail
and the RAM is
http://geizhals.at/de/g-skill-dimm-kit-4gb-pc2-8000u-cl5-5-5-15-ddr2-1000-f2-8000cl5d-4gbpq-a286178.html
The scenario is like this:
The RAM works in Single Channel mode, installed on Dimm 0 and 3. But as
soon as I try to install it on Dimm 0 and Dimm 1 (for Dual Channel) it
simple do not POST, just the CPU Fan power up but no signal on screen.
Additional tips
BIOS is flashed to the latest version.
I tested with another motherboard, same problem.
I tested with another pair of identical RAM, same problem.
Also tested with another PSU, same problem.
That means, I'm getting crazy! :)
Could it be the 2.1v rating? Or because the RAM is PC2-8000?