and the total cost for apu with those dual ranked 2400 MHz DIMMs would be ....

Ranked memory by the way are double sided memory sticks, this the RAM ICs are at both sides of the APU. Dual rank it seems is some 6…7% faster than single rank clock vs. clock in 3D gaming with Kaveri APU. Basically the performance is best with two DIMMs (dual-rank) installed that are running a high memory clock frequency (say 2400 MHz).

An unreliable way of checking whether a DIMM has two ranks is by looking at whether both sides of its PCB has DRAM chips; a reliable way of doing that, however, is using software such as AIDA64 (MotherboardSPD tab), which lists out number of ranks, and number of banks per rank; and of course, looking up the specs sheet. The reason visual inspection of a DIMM is not reliable these days is because of the way modern DRAM chips are designed. There are instances of DIMMs with chips on just one side, yet 2 ranks

Aren't most memory sticks dual sided nowadays, anyway? (If I understand correctly, "dual-ranked" means "double-sided.") I can get 2 x 4 GB sticks rated at 2400 MHz for €80, while the same amount of memory at 1866 MHz costs €74. So, at least for me, the added cost is worth it if the performance difference is big enough — which I think it is.

I wonder if those APUs can produce minimal variance in frame latency, as I read from some site, the frame latencies are all over the place and that translates to a not smooth gaming experience.