In my experience, ram sticks try to do what you tell them to do. It's like overclocking a processor and pushing it beyond its specs. When installing to different kinds of sticks however, both sticks are forced to run at the same speed and the slower one usually causes stability problems first, which limits how far you can push the ram. In the non-overclocking world, I think the bios just chooses the slower speed to run at automatically, which slows things down.
OK, then I should have no trouble using them... I imagine that I cannot use them in a dual channel configuration, right?
No, you can still set them up in a dual channel config, just make sure you plug the sticks in the correct slots.