Not all of them, I'm afraid. In fact, I tried DDR2-800 rated memory on one of my friend's systems that supported only up to DDR2-667 memory - and that system refused to even POST at all. Same thing happened with another set of DDR2-800 memory. (No such problems at all whatsoever with any of the DDR2-667 rated dual-channel kits I tried.) And when I ran CPU-Z on another friend's system that officially supported DDR2-800 memory, I've discovered that the two DDR2-800 kits I tested have SPD's programmed for a fixed, DDR2-800 speed (and no programmings at all whatsoever for slower speeds). As it turned out, that friend's motherboard required memory whose SPD programming specifically included settings for DDR2-667 and DDR2-533 speeds in order to even run at all.
Thus, the motherboard's (or CPU's) memory controller must officially support DDR2-800 memory in order to use the two particular models of DDR2-800 memory that I tested.