|You didn't list any other system specs? The proper RAM configuration depends on the motherboard & CPU. For example, almost all boards support running the RAM in dual channel mode these days...if you install 3 sticks, you'll force it to run single channel mode.|
Also, if you install the PC6400 (DDR2-800) along with the PC5300 (DDR2-667), all the RAM will run as PC5300.
And lastly, if you have an Intel system, there's a certain CPU:DRAM ratio that *should* be maintained to provide best performance. If you have a Core 2 system, that ratio is either 1:1 or 1:2. If you have a P4 system, the ratio is either 3:4 or 4:5.
If you have an AMD system, there is no ratio to be concerned with...you should be running PC6400 with all non-Phenom based CPUs or PC8500 for all Phenom based CPUs. Anything else hurts performance.