Getting the right DDR memory combination for a PC

Here are the general rules for combining DDR memories.

1. DDR, DDR2 and DDR3 cannot be combined in a single motherboard.  Some motherboards support 2 or more DDR memory types.  Some motherboards sometimes have slots for both DDR and DDR2 or slots for both DDR2 and DDR3.  However, these motherboards always warn against using both types at the same time (i.e., you can use DDR only or DDR2 only but not both at the same time).  Ignoring this warning can lead to damage to the board and memory modules.  Motherboards support multiple RAM types as a way to entice users who may want to replace or upgrade their motherboards but cannot afford to replace their old-technology RAM modules yet.

2. RAM of the same type but of different speeds can be combined.  The system will adjust its bus speed to match the speed of the slower RAM.  The faster RAM will run at the speed of the slowest RAM.  An example is combining a DDR2 333 MHz and a DDR2 400 MHz RAM. The DDR2 400 MHz RAM will run at 333 MHZ to match the speed of the slower DDR2 333 MHz RAM.