This is a little strange, I did a quick search at Kingston and they didn't list any 667MHz modules for that board, but Crucial does. What is the model of your RAM.
I tried to find the manual for your motherboard but ran out of patience with the Intel web site, check and see what it says about running a single RAM module in single channel.
Have you defragged the hdd, and how much free space do you have on it.
Have you looked to see what applications are running and tried to disable any that aren't necessary?