with RDRAM there was PC600 PC700 and PC800, currently PC800 is what you get buying online, Dell and other manufacturers used to not specify what they gave you and usually sent PC600 due to it being poorer quality and therefore cheaper. Currently PC800 comes in low density and high density sticks, you can tell by how the stick is set up, on older sticks it has ram with heatsinks on both sides of the stick for 256megs, newer 256meg sticks come packed onto one side like the old 128s used to. However you can mix the two on boards as the only difference is the physical architecture of the stick, not the access rates.