cin19 :
If the pc will not boot when using the two different single stick ram, nothing you can do, exchange the new one. Get the same speed and timing RAM, because when you mixed the two different RAM, there is no guarantee they will work together.
My pc boots, but when i open a browser or watch a video or launch a game it reboots, i checked if it was the gpu and it wasnt, i checked the power supply and it wasnt but when i removed the 667 mhz ram and stayed with the 800 everything worked fine, i read that you need to nake a configuration in the BIOS to use rams with diferrent speed and timing i dont know if this is true
____________
As I mentioned above, increasing the DRAM voltage can help as can an increasing the MC voltage, that's why I asked what mobo and CPU, need info to be able to help