You don't often see performance scaling as clean as what Phoronix saw when testing the effect of memory speed on AMD's A10-7850K.  Pick any result and you can clearly see a smooth increase in performance from DDR3-800 to DDR3-2400.  The only time that increase seems to decline slightly is between DDR3-2133 and 2400MHz, with some tests showing little to no increase between those two speeds.  Some tests do still show an improvement, for certain workloads on Linux the extra money is worth it but in other cases you can save a few dollars and limit yourself to the slightly cheaper DDR3-2133.  Check out the full review here.

"Earlier in the week I published benchmarks showing AMD Kaveri's DDR3-800MHz through DDR3-2133MHz system memory performance. Those results showed this latest-generation AMD APU craving — and being able to take advantage of — high memory frequencies. Many were curious how DDR3-2400MHz would fair with Kaveri so here's some benchmarks as we test out Kingston's HyperX Beast 8GB DDR3-2400MHz memory kit."

Here are some more Memory articles from around the web:

Memory