memory mix up

Hi

I ordered 8 gig new memory 2 x sets of
- Kingston 2 x 2 gig modules - DDR 2 PC5300 667MHZ
for my 4 core intel mac tower.

looking at my existing memory which I have taken out.
I had a mixture of

apple 2 x 1 gig modules DDR 2 PC5300 667MHZ
and
a mixture of
- Kingston 2 x 1 gig DDR 2 PC5300 555MHZ
- Micron 1 x .5 gig DDR 2 PC5300 555MHZ.

my question is
can i mix different clock speeds? [667MHZ + 555MHZ]
I am guessing not
I alway suspected my memory was not right.

what machines would the 555mhz be suitable


any help would much appreciated
mikef