As CoW]8(0) already said.

If you use them only at standard speeds you won't see or feel any difference,
only in Benchmarks like SuperPi oder Aquamark3 Memoryperformance makes a quite big diffrence.

The BallistiX 8500 seem to have the best selected Micron Chips,
cause they often are highly overclockable and reach better clocks and timings than any other DIMMs.

Upgrading from 6400 to 8500 is only useful for benching.