The ASUS M5A78L-M/USB3 does support all of the current FX CPU's, IF the BIOS is 0411 or higher if not it won't boot. Next, the FX CPU's IMC is rated stock for DDR3-1866 IF the RAM has a 'SPD' JEDEC of 933MHz, but not if the kit's 933MHz is XMP. If using XMP kits the RAM runs at the highest available non-XMP up to 933MHz (DDR3-1866). To correct this the RAM must be manually set in the BIOS.
* My assumption is you're confusing AM3 100 and 200 series (DDR3-1066) with the FX's DDR3 1866MHz. *
You can use DDR3-1866 with 4GB/stick density. The FX can also run faster than DDR3-1866.
ASUS M5A78L-M/USB3 - http://www.asus.com/Motherboards/AMD_AM3Plus/M5A78LMUSB... 4 x DIMM, Max. 16GB, DDR3 2000(O.C.)/1866(O.C.)/1800(O.C.)/1600(O.C.)/1333/1066 Hz ECC, Non-ECC, Un-buffered Memory
Dual Channel Memory Architecture
* Refer to www.asus.com or user manual for the Memory QVL (Qualified Vendors Lists).
* Due to OS limitation, when installing total memory of 4GB capacity or more, Windows® 32-bit operation system may only recognize less than 3GB. Install a 64-bit Windows® OS when you want to install 4GB or more memory on the motherboard.
* AMD FX Series CPU on this motherboard supports up to DDR3 1866MHz as its standard memory frequency.
* AMD AM3 100 and 200 series CPU support up to DDR3 1066MHz.