For your needs you need a balanced system, hard drive/ram/cpu/gpu are all equally as important working with the apps you mentioned. I can get a single Premiere file to 28 GB RAM alone so overkill for this type of build, definitely not. Never go with just 2 drives it'll slow everything down, you want a dedicated scratch disk and media drive, and export drive to balance it, resulting in faster workflow and lower render times than going with a more powerful CPU. Except in this case the CPU is about the same as the Xeon in this build except no overclocking or integrated graphics.
Edit: Andy, I have to agree here, WD is a great brand though I don't think he will run into any issues, hard drives don't really fail unless you physically harm them from my experience.