The primary mistake you are making is leaving the GTL ref's on disabled, even with VTT point of 1.38 and suitably applied GTL's you would see more success. The GTL's need to operate within a margin of VTT, typically around 1-2% within 67% of VTT, anything below or above this margin will likely fail to hold steady (unless you have a real freak processor)
A side effect of Cas 4 is that VNB and Vdimm will need to be higher than Cas 5 clocks. Check Kris's article on CAS related overclocking margins over at the Tech Repository.
Cas 4 on DDR 667 ram, which is not binned as Micron D9GKX, will need quitwe a voltage bump in some cases to scale, bear this in mind...
The 266 strap and divider combo's will operate Performance level 6 on this board upto around 463 fsb (266/667 divider). The 333 strap will give a margin of around 10-15 fsb over the 266 strap for absolute stability (333 strap loses latency and fsb overhead is a little low to claw back the deficit), however I feel that the 266 strap is the sweet performance ratio for Quad core clocking on this board (if not benching for extreme cpu mhz).
1.5vnb will get you 400fsb ddr 1200 Cas 5 on this board. Cas 4 gives the NB a harder time, so one has no choice but to bump VNB to provide suffcient overhead for the lower latency. 450fsb will require 1.61-1.65 VNB @ Cas 5 , with suitably low VDimm on good ram. 400 Fsb should come with VTT of around 1.50v (using the appropriate GTL ranges) and upwards, or less if you are prepared to put in the time to study how much VTT/GTL your setup can get away with.
It's just a matter of you picking a range of performance and voltages within the boards abilities at a point you are comfortable with (there will have to be compromises in both directions.
regards
Raja
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