I have 2 sticks of 1GB ram I just bought. They are both 6400 800 Mhz 1GB sticks from AMPX. My old 2 memory sticks were 4200 sticks of 512mb I just tried to put my 6400's in and my screen wouldnt work and would stay blank... I have an Intel Pentium D CPU 2.66Ghz. I was told that 6400 would work (just not to its full potential)... Is this due to compatibility issues?

Can't help you without knowing the make/model of your board.

In your case, probably not as you have found out. If your computer was handling 533mhz memory (4200) which the Pentium D's did, then 667mhz is probably all the higher the computer will recognize. Give us the manufacturer of the motherboard, the model number and we can be of more assistance. Not all Pentium D's were made to support 800mhz, and not all motherboards were either. Some motherboards can be extremely finicky when it comes to memory. Thats why manufacturers have recommended memory lists. They are all DDR2, and may physically fit the slot, but as you see, may not be compatible. Higher ram MAY work in lower systems, running at lower speed governed by the lower systems slower bus speed, but it's not a sure thing. Mn Remote Pc