First, if you are still using the stock cooler that came with your processor, you should buy a better cooler before playing with the voltages. The one am using, it costs around £16 from www.cpucitystore.co.uk.
Second, if you are talking about CPU_FSB jumper near the Northbridge, it has to be in the default position (FSB 333/266) according to the manual. (I believe is in that position otherwise you wouldnt have been able to overclock).
Third, try to test each part of your system step by step to see how far you can go with each individual component. For instance you can test each stick of memory by having the FSB running at 166 (the processor is not overclocked) and increase the frequency of memory to 120% of FSB (200) single channel mode. Run some memory tests or some benchmarks software to make sure the memory runs stable. Then test the processor (overclock the processor but not the memory) etc.
Consider, as well, that if you dont provide proper cooling for the processor the worm air coming out of your heat sink blows directly to the memory which might contribute to memory instabilities.