One more thought. There are only a few good reasons for adding memory rated at a higher speed than the pc it's going into.

1) the higher speed memory is cheaper. i've assume this is the case here. this frequently happens when the older speed is no longer used in new pc's.

2) you're planning to scrap the old pc *soon* and want to use the new memory in your new pc.

3) you are going to overclock. (added for completeness. please don't try this at home.)

Merely adding higher speed memory will not improve your performance. If one of these reasons doesn't apply, you'd be wasting your money.