While technically, it is only supported at 1 DIMM per channel, you should be able to run 2 DIMMs per channel without trouble. I'm running 2 per channel in my i7 (which has the same warning), and it's running flawlessly.
@cji I was thinking of 4 x 2Gb = 8Gb ddr3-1600. Is that your configuration, and does your OS see all the memory, does it use it ? (Sorry, im noob ..) I've already bought the crucial sticks, so this is a bit worrisome !
As long as your OS is 64-bit it should see all the memory. The 64-bit version of Windows handles large amounts of random access memory (RAM) more effectively than a 32-bit system. A 32-bit OS can theoretically use up to 4 GB of RAM, but 32-bit versions of Windows Vista and Windows 7 see a maximum of 3.12 GB. With 64-bit Windows 7, you can use the full 4GB RAM.
Well, cjl had the same warning on his mobo and he isnt having any problems, so you shouldnt be having too. My mobo manual says i should run my ram at 1333 mhz but atm i have 2 sticks running at 1700 mhz and my system is completely stable. ^^