Which RAM to use with AMD Phenom II x4 955?

You need to check the spec of your mother board. If its a high end board like Asus or Gigabyte it will take the 1600...however...it will automatically default to 1333 (the speed of the FSB) you can use the "tweaker" that comes on the mother board bios to raise the ram speed to its 1600 potential. If you have a "factory" mother board like on a Dell or HP, then there is no "tweaker" but may be able to use a third party program like CPU-Z to do the job.