You would want to set the RAM @ 166 or 133 depending on how high you get the HTT (CPU buss speed) and set you HT...
HT default = 5x200=1000
4x up to 250MHz
250Mhz-333MHz use 3x
By setting the RAM speed to 133 or 166 it helps to allows the CPU to be OCed without the RAM's limitations holding the CPU back. Start with 133 and see where you get.
To OC the RAM to determine it's top speed, set the vdimm (RAM voltage) to it's highest safe voltage, 2.8 should be fine. Then underclock the CPU with a 6 or 7x so it won't hold back the RAM. This will give you the information you need to figure out how high it can clock so you know your "stable" max speed.
Try this for quick results first ...
A quick test complements of ncsa
When you boot to memtest it auto starts - pressing C 1 3 x <enter> <enter> starts to loop the test where x is the test number.
Once you have done this it will only loop that test until reset or configured for another test.
Tests 5 & 6 are quicker but 8 is more accurate once you start to settle down the OC and the timings / Vdimm.