Can RAM of different frequency be used together?

Hello friends!

I am having 2 GB DDR2 @ 667 Mhz in my PC. I am planning to add another 2 GB to it but as 667 Mhz is not available, can I use a 800 Mhz with my present 667 Mhz.

Is this going to create any problem. If not, does this mean that both will work at 667 Mhz or will perform individually at their individual speeds?

Thank you.