Have you tried changing NB GTL? This was vital for me but i'm not sure if it's because i'm on a Quad. How high vdimm? Here is some of the info Raja sent me, some of it might be useful:

The trade off in the Samsung based parts is that they don't really like anything below CAS 9, without requiring judicious levels of voltage. Running CAS 7 curtails the prospects of outright memory bandwidth (hence FSB) on these chips.

To add some stability into the mix you can also set 6,8,3,6 for the R2RD (Same), R2RD (Different), W2RD (Same), W2RD (Different) settings, though at a bit of a performance hit

Make sure you use slots 2&4 for the memory for a start.

SAmsung memory does not like high Vdimm for prolonged periods - I never use more than 2V on it as I doubt it qould last long. In fact, while we're on the subject I have their 1GB modules here and they actually clock worse over 2V Vdimm - go figure.. If running at CAS 9 where the modules are desinged to run you should fiund yourself getting returns at just under 2V. this is to do with coming into Jedec spec and also gearing up for the on die IMC of Nehalem which demnds the use of lower reference volatges such as Vdimm VTT.

Having to skew a signal depends mainly on 2 factors, the first is due to a mistmatch in motherboard PCB trace lengths. The second is down to a heavily capacitive load stressing the MCH signal output devices to such an extent that waveform slope/hold/fall times become distorted and affect slot to slot data acquisition times. The heavier the capacitive load of the memory modules, the more likely this situation is likely to manifest. When it comes to the Samsung modules (even in 2GB format), they represent a very easy load for the MCH to drive, so generally do not need signal skews when operating around the 1800Mhz mark

Sounds to me like the clock skew values are what you need to be looking at, but i have no experience myself with memory clocks that high.