Memory size and speed are critical for only a few programs such as databases, video or audio processing, Mathematica, and similar applications that use the functional programming model. Because it's usually not critical for performance, memory is always the hardware component that is least well documented by PC manufacturers. It is also where they try to save a few bucks. Default memory speeds are always very conservative because memory issues are so hard to isolate. This means that almost any memory over-clocking will get you a measurable improvement for very little risk. Once you have the additional processors to enable parallelism, memory is the primary tuning knob that you want to adjust in order to speed up Mathematica. 32 gigabytes of memory is a goodly amount by today’s standards, but you might actually be happier with 16 gigabytes of slightly faster memory that can be over-clocked to improve buffer transfer speed and random access speed. 1866 is fairly slow memory by today's standards. The newest DDR4 memory is now available at 3300, but that won't work with a Z97 motherboard. If you’re interested in a few, or a lot, of over-clocking options then read on. The Z97 chipset over-clocks well and "enthusiast" motherboards such as the one you mentioned have an overwhelming array of over-clocking settings that you can tweak until you run out of patience. The memory and motherboard makers will each have forums with posts that discuss dozens of settings that will work, but may interact weirdly. They can advise you on the free benchmark programs that you can use to measure results. Tweak memory before you tweak the CPU. Be VERY cautious when applying more voltage. Go slow. Measure the result. The last time I built a serious machine I spent a week tweaking memory, but the results were about 150% over default memory speed. If that sounds like too much work, and it is a lot of work, just go with 32 gigabytes of slightly slower memory and apply a few of the simplest memory setting tweaks. Sometimes this can be as simple as clicking a BIOS option labeled AutoOverClock enable.
Note: Once you have your system be sure to modify the Windows swapfile size from "system determined" to a pre-allocated swapfile that is either 1X, 1.5X, or 2X your total RAM size. It can be split over 2 disks.
Personal note: I am a rank amateur with Mathematica, but I do hardware/software performance tuning for a living. Memory constrained applications were common 35 years ago, but are now very rare.