So I have a HP Compaq DC7900 CMT desktop, that came with what appears to be the two 2GB sticks of DDR2 667MHz DIMM RAM. (It has four RAM slots.) I went to the local computer parts place and got another two sticks of 2GB RAM, which ended up being 800MHz RAM with slightly faster timings. From what I'd seen on the computer manual the MC and mobo can handle this fine, and it did for a long time before I noticed when I started to use the 6-8th GB of ram, stuff would start to freak out. I ran Memtest86+ and after a while I figured out that that stick had one bit that was defective, so I tossed the stick and put the other in a bag. The computer has only the stock sticks of RAM now and I was curious if there's any real serious issue with mixing and matching RAM sticks from wherever. I have seen both sides, that it will destroy your mobo and hardware, and that it will do nothing, but all my computing life I've mixed and matched weird RAM (my media server laptop has 3gb in two SODIMMs, one from an apple laptop) and never had problems. tl;dr (as well as question) would it be a bad idea to get another 2GB stick and go back to 8GB RAM? Thanks for reading! First new topic for me.