One of two problems with that;
1 - Can't have RAM sticks of different speeds together on the same computer, cuz the computer can only run your RAM at one speed. So if the speed specs of one stick are different from the other, it gets all confused and bad things happen :lol:
2 - Even if you pull out the old, slower RAM and drop in the new, faster RAM, you gotta make sure that your motherboard supports the new RAM speed, and that it's compatible with the CPU you're using, otherwise you'll have to use a RAM divider in the BIOS.
Hehe, hope your PC dudes know their biz 8)