|As Fingers mentioned above running RAM in dual channel mode is best, if possible. Dual channel increases memory bandwidth, which is advantageous in some instances.|
In your case the benefit of installing new matched RAM modules may not justify the expense. In the future keep it in mind.
To further comment on Finger reply, installing the slower RAM in the first slot may allow both modules to run at the slower speed. Conversely, installing the faster RAM in the first slot will force an overclock on the second module, which may not work.
Of course, the RAM must be compatible for your computer.
message edited by OtheHill