If you’re looking to upgrade your current system’s CPU, and will be shooting 4k, you should be upgrading to a Xeon processor (or two). You’ll also need to swap out the motherboard as well.
In regards to your hard drive question: don’t look anywhere else except moving to RAID 0 with 4 of the fastest SSD drives you can afford (Samsung 840 Pro or equivalent). You might want to look at vendors that provide dedicated RAID systems, like G Drive for example.
Dave