It means you can only run two sticks of 1066 is dual channel.
TBH, I can't see why you couldn't just force it to run 1066 dual channel by overriding the auto settings. Just go with a different motherboard just in case, I'd take a 790FX or even a 790GX over a 780a anyday, even if it didn't leave me with the option of running SLi later.
EDIT: The 980a is just a rebranded 780a. The 980a was originally intended to be the name of the NB of Nvidia's DDR3 boards (Since the memory controller is located in the CPU, the north bridge doesn't change at all 780a = 980a). The board you linked to is still DDR2, and they're calling it a 980a. This is just a big fat fail for Nvidia's rebranding.
AMD Phenom II X4 955 BE @ 4.0GHz + TU120E lapped - MSI 770-CD45 AM3 - 2x2GB OCZ DDR3 1333MHz - Sapphire HD 4870 - Samsung Spinpoint 500GB 7200RPM 16MB cache HDD - Tagan TG600-BZ Piperock - (Currently open test bed) - Windows Vista Home Premium 64bit.