You can install any combination of DDR2 speeds together
DDR2, now being phased out by DD3, was the industry standard for several years. Offering twice the speed of DDR and increased throughput, it could be found in both PC and Mac applications. RAM speed is measured (somewhat confusingly) in Mhz and data transfer speed. For example, DDR2 800 (400Mhz but because DD2 is "double pumped," the RAM clock is multiplied by 2, thus a theoretical 800Mhz clock speed) is the same as DDR2 PC6400 (the 6400 referring to the maximum bandwidth possible with that particular RAM). Although it seems confusing, the simple rule of thumb when dealing with DDR2 speeds: you can mix and match speeds to your heart's content. However, your RAM will only run as fast as your slowest stick of memory.
Instructions
1. Power down and remove all plugs and cables from your PC. Place the case on a flat surface that you're comfortable working on.
2. Unscrew the screws holding the side panel of the case in place. The location of these screws vary from case to case. Typically, they are on the back of the case, spaced evenly apart. Consult your owner's manual if you're having trouble finding them. Remove the side panel of the case.
3. Locate your current DIMM slots. If you are replacing the RAM you have installed, remove it by pressing down gently on the clips on either side of the DIMM. Once you've detached both clips, the RAM should come easily out of the slot. If it doesn't, make sure the clips are completely free from the stick of RAM.
4. Line up the notch on your new stick of RAM with the notch in the DIMM slot itself. Press the stick firmly into the DIMM slot until the two clips on either side of the DIMM "click" into the notches on the sides of the RAM module. Note that the RAM will only fit into the DIMM slot one way.
5. Replace the side panel of your case and reconnect any cables and plugs and power on your PC.
Tags: DIMM slot, side panel, clips either, clips either side, DDR2 speeds