Hey everybody!

I noticed that on some RAM modules that it has a "U" after the PC133 or whatever number. In this case, it said "PC133U", so I was wondering, what is the "U" for? What does the "PC133" mean? Also, I've heard that it's better to have the same "density" of memory modules? Can someone explain this in a simple to understand way? Is it better to use two RAM modules of PC133 than a combination of PC133U and PC133?

I'm basically looking to understand how RAM works. I've looked at the Wikipedia page already and had questions so I came here.