As above poster has stated. Also, if you use two or more DIMMs with differing speeds the BIOS always declocks the faster DIMMs so they all run at the same speed. Therefore, you won't see the benefit of 1866 DIMMs.
Now, that being said, in reality, you're very unlikely to actually note any appreciable difference between the two clocks speeds from normal usage. 16GB is more than enough for typical usage and gaming. Unless you are running really high-memory usage applications you shouldn't need 24GB
It could in fact actually make your system less stable - as you have two completely different sets of DIMMs running at the same time if you use all four. It isn't such of an issue these days but worth noting nonetheless.
