My customers typically have Win7 PCs in their offices, from various manufacturers. Recently I ran into problems at two sites where they wanted to add 4GB RAM. They both already had a single 4GB stick of PC3-10600 RAM, but all I had with me was generic 4GB PC3-12800--and I could not get that to work. I believe it was an Acer that did BIOS error beeps and nothing else. And it was a Lenovo that did nothing, i.e. no beeps, no video, no nothing other than powering up. I am not absolutely sure, but I feel confident that both were capable of accepting 8GB total. In fact, with the Lenovo, when I finally found another PC3-10600 stick, I was able to boot it up like normal, showing the full 8GB.
I thought faster RAM was backward compatible, and should have worked--but clearly I was wrong. I'm hoping someone can explain "the rules" to me so I will know how to deal with such situations. TIA