DDR3 is more forgiving than DDR2 was, so that's not much of an issue nowadays.To be perfecttly honest its best that you don't mix and match different memory brands.
I did that in the past with my old intel builds and the results always had windows or the whole machine getting out of sync.
You'll never notice a slight increase in memory speed like that, that it would make a dramatic difference in anything other than benchmark scores (if that).The crucial one would be better for gaming, you'll noticed the difference if you do online matches that are intense.