That really depends on the CPU in your machine. It will only make a difference if the bus speed of the CPU can be multiplied into the ram speed (so if the FSB was for instance 200Mhz, 800 DDR would be a division of 4, if the buss was 166 then 667 is fine). All assuming your motherboard is setup right to do it. Its a murky puddle of configuration, have seen motherboards that do this right when using 2 sticks and slowing down on 4 sticks etc....
If you are just going to make this change through to gain, you probably wont notice the change over other changes like more memory, zipper video card etc...