generally you can, it depends on the system. there was a triple channel memory system at one point. But if yours is dual channel, then it should work. (the specs are so similar on these, I think it will work no matter which way you do it, you just have to try.)

Usually the manual for the motherboard or the system will tell you which way they want you to do it, but you won't hurt anything by experimenting with it and running some memory tests if you don't have or can't find the manuals.

Even if the DIMMs in a channel are different, the system firmware should be smart enough to use the lowest settings, highest number of wait states necessary to make it work. Of course, you are depending on the ability of firmware programmers to make that work properly. And that isn't the idea setup either.

edit: the specs are so similar on these, I think it will work no matter which way you do it, you just have to try.

-David