Legit Reviews did a marathon testing run on Llano to see just how dependant is on the memory subsystem.  They took a variety of 4GB DDR3 DIMMs, at 1333MHz, 1600MHz and 1866MHz, which they tried in pairs as well as with all four memory slots filled.  Their testing shows that 1600MHz really is the sweet spot for Llano, not as expensive as the 1866MHz kits and in some tests noticeably faster than the 1333MHz DIMMs.  Also of note was the consistently better performance with 4 DIMMs as compared to a pair.

"We found that the memory bandwidth ranges from 13GB/s to 16GB/s with typical dual channel memory kits on our AMD A8-3850 APU and Gigabyte A75-UD4H motherboard. This difference doesn’t sound that great, but a 3GB/s increase when dealing with 13GB is a very nice 23% performance increase. Most of this performance gain is seen when moving up from a 1333Mhz to a 1600MHz memory kit is where you’ll see the largest performance jump. For example 2x4GB 1333MHz CL8 memory kit was able to hit ~12.7GB/s memory bandwidth, but the 2x4GB 1600MHz CL8 memory kit has ~15.0GB/s of memory bandwidth, which is a very nice 18% improvement over 1333MHz…"

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Memory