The Phenom II memory controller in the AM2+ CPUs is made to handle only one dimm per memory channel. I'm not sure if this was changed in the AM3 CPUs, but I doubt it was. I have they are more tolerant of clocking with 4 dimms. I can even run 4 dimms on my old Phenom II 940 with mixed matched memory as long as I run pretty lose timings and give the memory decent voltage. Unfortunately I still got an error here and there under high loads so I just went back to running the 8 gigs of ram I have at 800mhz.