umm.. you really cannot use DDR and DDR2 together. first off, they dont use the same slots. second, if your mobo is a bit old and have slots for both DDR and DDR2, if you put in one DDR and one DDR2 RAM, your pc will never start. it will fail a system check
in thread starter's case though, DDR2 533 and DDR2 667 are involved. they should work well together but your 667 will only function at 533 speed. kingston and PQI are not that picky (Unlike other brands like geil, corsair, etc)