I will add my 2 cents for a LightRoom based computer...
Get the highest amount of cores and clock speed you can if you plan on exporting or batching a lot of pictures. An i7-4790K is great when you start Exporting or batching. You really won't notice the speed of this processor anywhere else. You could get a much "cheaper" processor and almost not notice a difference, but it's all about the Export and batch functions.
Primary/OS drive should be a SSD.
Secondary Drive can be as slow as a 7200RPM hard drive to store all of your RAW and exported pictures on is fine. When exporting, your processor will always be the bottleneck, not the hard drive, even at the old 7200RPM speed. Of course importing or copying to this drive from your camera's memory card will take longer, it won't have a speed affect once working in Lightroom.
This is true if you're doing large batches daily or at least really frequently, or need them done fast for jobs... but imo, not a good reason to dish out loads of extra cash.
My nearly 5 year old Phenom II 1090T with 8GB of 1333mhz RAM will import or export 800 shots (though admittedly 12mp shots usually since I use an a7S) in about 5-10 minutes (I've never timed it, because it's never been slow enough to bother me). I just click import/export and watch a Youtube video, then it's good to go... worth saving the extra $200+ imo.


