If all 6gb are recognized and working it is good.
That said, the speed or timings do not much matter on a X58 based system. The difference in real application performance or FPS(vs. synthetic benchmarks) is on the order of 1-3% between the fastest and slowest ram. The integrated memory controller on nehalem processors is so good that it can keep the processor supplied with ram data from any speed ram. Do not be seduced by wonderful synthetic benchmarks unless you are a competitive overclocker.
I would keep it simple, and let everything default. The ram speeds might be slower, but auto will give you beter timings.
what if the memory is not reconized, i have 6 gb but it only shows 1 gb available y tried 2 sets of memory and the board still shows only 1 gb i have the sabretooth board and a icore 7 920