If you have them, give them a try...If they play, they will prob run in flex mode, or effectively you'll have 4GB in dual channel and the odd 2GB off the 4GB running in single channel
No. You want to have the same speed. Basically, you want the same exact stick. Now, it is possible that it could still function, but it may not work. Also, you want to make sure you have 64-bit, if not, you will only have 4GB usable.
If you have them, give them a try...If they play, they will prob run in flex mode, or effectively you'll have 4GB in dual channel and the odd 2GB off the 4GB running in single channel