Part of the curse of being a hardware enthusiast is figuring out the right time to upgrade. The desire to upgrade is driven in large part by ego and boredom, and that can cause us to upgrade prematurely because we want to have the latest hardware to play with. And then a few months down the road, the itch returns and we'll be salivating at the next prospect..
At any rate, your Sandy Bridge will remain viable for years, so there's really no "need" to upgrade it at this time.....other than ego and boredom as I said earlier.
Yep. It's fun though.
I just dismantled my 4790K for a haswell-E build...seems silly, but a lot of things led to that decision. My 4790K didn't exactly win the silicon lottery, and I was never happy about that. Plus half the die is dedicated to graphics that are absolutely useless to me. Also, for the first time, I bought one of those fancy $200+ Asus motherboards and regretted it immensely - all the extra addons like sound, sata ports etc are addons you'll never use at best and PCIE lane soaking junk at worst. I also run dual GPU, and Z97 won't have enough lanes to run dual cards and an x4 NVME SSD, which are right around the corner.
At the end of the day, a 5820K is what, $20 more expensive that a 4790K? I'll pay $20 more for 2 extra cores I can actually use vs. useless graphics silicon. A basic X99 mobo costs the same as that fancy ROG mobo, except instead of upgraded sound and other useless things, I get stuff that's actually useful, like quad channel memory and more PCIE lanes. Now I can run 16x/8x GPUs and still have 4 lanes left over for a 32gb/s SSD when that time comes....and it's not like x99 is short on SATA or USB ports either.
So if you're already shopping at the top end of the Z97 platform, it basically comes down to the memory being more expensive, and while that's still an issue, for 16GB the premium really isnt that bad. You're already spending well over $500, what's another $50 at that point? Obviously the equation changes if you're shopping around the i5 range, but I think the bang for the buck is a lot better for a 5820K setup vs a 4790K.