Honestly on Intel platforms RAM speed doesn't make much difference. Anything above 1600MHz is getting you VEEEEERY little benefit. It's going to come down to what you use your PC for. If you're heavy into designing, rendering, etc then just find some cheap 12GB 1600MHz kits and throw in a couple of them.
If you're a gamer, then find a single 12GB kit and throw that in.
At this point I shop for memory based on colour coordinating with my other components, not based on performance.