DRAM - Can I mix DDR, DDR2 and DDR3 modules in my PC?

No. DDR, DDR2, DDR3 are different. You must select the correct type to match your system. DDR, DDR2 and DDR3, DIMM and SODIMM modules have notches in different places to prevent insertion into a incompatible system. See our memory guides; DIMM or SODIMM. Below are some basic differences.