Many things to learn.
1. RAM voltage increases and timing changes can help improve the speed and have a lot of effect on the OCing, even at moderate levels. Your DDR2 667 will be even more a factor and may require using a divider or under clocking it.
What exact RAM is it?
Post the "SPD" tab of CPU-Z which you can get @ www.cpuid.com
2. Also relative to the RAM is the memory controller or HT bus.
It always bases at the default CPU bus speed of 200
MHz regardless of what base speed the RAM is running at so as the CPU bus speed increases so does the HT bus speed increase equally.
As the CPU is OCed, the default of the HT also rises and may require a reduction in the 5
. A reduction to 4
should work with a CPU bus OC of up to about 260, beyond 260 drop to 3x.
BTW, some BIOS use "1000" to represent 5x, "800" for 4x and "600" for 3x
3. The CPU...
Stress test with OCT or Orthos and keep the full load temp below the very low 50c range.
Check the temps with CoreTemp ver 0.95, or 0.94, some people have had issues with ver 0.94
Only increase the CPU voltage or vcore a little at a time as necessary! What is necessary, try OCing with no increases and if you get either flaws on your screen, errors in OCCT or Orthos or if you pushed way to hard to fast no BOOT.
Move up slowly, take your time and you wont likely bet to the point of BOOT problems!
Check the full temps, 50c is the recommended max temp but low low 50c is still very safe
CPU voltage... mis 1.5 range is getting to the level of pushing it, especially if you don't have a very good HSF to cool it!
RAM, 2-2.1v is the likely safe max for over volting the RAM!