Why use a 2600K unlocked CPU version and not overclock it? The cheapest and fastest way to improve your system is by overclocking to 4 GHz or even a bit higher if you can keep the temps down. No sense in getting an Areca card in this system. Your raid0 WD array is fast, but you need to be meticulous about backing up your media.
The next step would be to go for an all new system based on a X99 mobo with DDR4 memory and an i7-5930K CPU or even better, but that will add up.
...although the intel flagship will remain the Haswell E series CPU for a while longer, until the Skylake E CPU appears a long time from now...the FIRST reviews of the NEW desktop Skylake CPU are INTERESTING. For a MUCH lower budget compared to Haswell E, it may be possible to assemble a solid performing machine based on the new 6700K Skylake CPU.
Even though it "only" has 4 cores and 8 "threads", on one website this morning I saw where it outperformed the "old" 6 core 3960X CPU in video encoding...which was intel's best only a few years ago. This is due to the MUCH higher clock speed on the Skylake....4.0 Ghz normal to 4.4 Ghz "turbo".
If you can manage to stay within the " lanes" of PCI generation 3 available......which may be LESS than the 40 available on Haswell E with a CPU of 6 or 8 cores... you could be OK.
So, if considering a new machine, compare the mid and high end Haswell E CPU based machines with what you can assemble with a Skylake based rig. The Skylake should be BETTER than the current 4 core Haswell 4790 for a similar price......especially due to a better MOBO and chipset.
Besides following cc's advice to overclock the HELL out of your current CPU, you can use the free MSI Afterburner program to overclock the " memory clock " on your current video card. I am doing this on my 765m video card on my laptop with great success and no overheating....improving performance about 25%. The desktop GPUs may not allow that much improvement because the laptop GPU is set STOCK to a lower memory clock to keep the heat down to a minimum for marketing purposes. However, it is worth trying.
A newer 970 may be a good idea,but, I am not sure if your current CPU warrants it....the 4GB video memory version would be advised....this new GPU may improve performance IF your CPU is overclocked to over 4.0 Ghz.
Lastly, you do not mention the make of your 120 GB SSD boot drive. Only the Samsung 850 Pro model performs well at that small size. The Samsung "EVO" and other company's SSDs at that small size have had problems concerning slow write speeds. This can be important as the Windows page file makes MANY constant and small reads and writes during normal operation. With Windows 7, you MAY place that page file onto a faster SSD,or, array to alleviate that problem. Starting with Windows 8, the page file MUST be on the "C" drive.and thus, you would need a faster SSD for the "C" drive. You MAY want to use the single 850 Pro as your boot drive, if your current 120GB SSD is inferior. If your current boot drive SSD is OK, you should ADD that single Samsung 850 Pro SSD to the current SSD array to make it even FASTER and larger. However, I do not know the limitations of your motherboard in regards to available SATA ports. There is no need for a separate SSD just for "media cache" when you already have fast arrays in your machine. Remember that for BEST performance, the drive which is "feeding",or, supplying your video files while editing needs to be your FASTEST drive ! Currently, your SSD RAID array is TWICE AS FAST as your media drive, which is the 8 TB RAID 0 array ! Performance may increase if you place ACTIVE media files and ACTIVE project files onto the SSD array....be it with 2 OR 3 Samsung Pro SSds.
Eric Bowen has reported on this forum in the past that the newest versions of CC Premiere contain software improvements that have increased the use of memory and have increased overall performance...especially with 4K. Windows 8.1 has also shown a performance increase over the use of Windows 7 with Premiere....keep that in mind.
With your current spinning hard drives you should :
1. Make sure " indexing" is turned OFF for ALL drives in the system.
2. Do not fill those drives to more than 50% of capacity,or, " fill degradation" will happen where each drive will eventually run at HALF the speed it did when brand new and empty. Your current speed of the 8TB array SHOULD be ~ 540 to 500 MB/sec. , ( 4 x150MB/sec minus 10% overhead ).
3. Faithfully " defragment" those drives to make sure they run at top efficiency.
4. MOST IMPORTANT !!!.....somehow, BACK UP the data on your current RAID 0 !!! The odds are HIGH that just ONE of these spinning drives will fail and ALL the data....including your original video and media files....will be LOST FOREVER !!!!
To backup an "image" of an 8 TB volume is EXPENSIVE....an "Enterprise level" HDD with 128MB cache and 205MB/sec speed at B&H photo is $600 !!! You CAN buy a "regular" 8 GB HDD for $260, though to backup that data....with less assurance than the enterprise level drive.
You can visit PPBM8.com to test your machine with the benchmark program written for Premiere Pro. It will identify problems and "bottlenecks" in a system and will provide a means to compare your current machine with others, in terms of performance.