my hz's that were 800 and 4-4-4-12 made it to 5.9, also a pair of corsair xms2 2x2gb at 5-5-5-15 890mhz also made it to 5.9, try overclocking your ram slight or, even better tighten your timings a bit.
i have some OCZ reapers 800mhz ram 4gb - got a 5.9 running 4-4-4-12 timings - also got one running 5-5-5-15... actually underclocked per-say to 1:1 my overclock...