I've recently bought my computer, so it's all pretty new.
My current specs are:
Corsair Vengeance 1600MHz 2x4GB
MSI 460GTX (A bit older)
Cooler Master GX 750W PSU
When I start up the computer, it boots fine with one stick of ram, no problems there.
The problems start when I put 2 sticks into it. Then it starts messing up and it's more luck that you actually get to POST or Bios.
The red DRAM_LED is lighting up too. I tried the MemOK! button, and that worked out fine, problem being it messes up the next time I start up too.
The early firmware revisions of the Asus X79 boards (all of them) have some SDRAM support problems. Use the one stick of memory to upgrade the firmware to the latest release then install the second stick. After that, get another matching set so you can use all 4 channels.