Help with RAM problem

I have a Dell Dimension 8400 Pc and have instaled 4GB (4 sticks) of DDR2 Memory which should run at 667Mhz. However when I look at the bios it states the memory is 555 mhz. Could anyone kindly let me know how I can achieve the correct speed of 667mhz?:confused:

I can confirm that all for ram sticks are the same, and that I have made no alterations to the settings/hardware in the PC.

Many Thanks!

Did you check with whom you purchased the sticks from that would be where I go would first. Then next check with newegg and see if they have your RAM specs and if they list more info on the RAM.