Any advice for pushing the memory controller a bit? Which voltages to adjust?

I have 4 sticks of DDR1000 memory and using the 333/800 stap I can get the memory up to around 990 (415) but that is still 10MHz under its rated speed. Any higher and I get errors in IBT. I've tried upping the voltage to CPU (q9450), VTT, NB and even the SB/PLL. Nothing helped. CPU, NB and MOSFETs are water cooled and stay nice and cool. When I changed the strap from 333/800 to 333/666 (thus 1:1) and the same settings I suddenly became stable.

Based on the above, does it sound like the memory controller is getting taxed due to the 4 sticks? If I wanted to go a bit higher, any way to get it stable or do I have to gank 2 sticks?