About to get a new cpu i5-6600k. I already have the i5-7400 but it bottlenecked my amd rx480 a lot more. First question is, is it worth the .5 ghz upgrade? Second question is that do I need to update my bios even though the 6600k came out in 2015 and the 7400 came out in 2017?
The 6600k is OLDER than the i5-7500. You are going BACKWARDS, rather than "upgrading".
Even so, you will be likely to see a SMALL increase in performance due to the 400mhz faster boost speed, but since the all core boost speed on the 7400 is about 3.3Ghz and the all core boost speed on the 6600k is only 3.5Ghz, that 200mhz is not enough to be worth the cost of the CPU UNLESS you plan to drop a manual all core overclock on there of about 4.5Ghz which will require substantially better than stock CPU and case cooling as well as a very decent quality motherboard with a good VRM configuration.
In sum, no, that upgrade is not a worthwhile one. You would be MUCH better off to find a good used 7700 or 7700k, or simply upgrade the whole motherboard and CPU platform to a newer one and use the DDR4 you alerady have.
If you DID decide to go with the older CPU, then no, you would not need to update your BIOS.
The 6700k would be a decent bump in performance from the i5-7400. You'd gain about 700mhz boost speed on an all core turbo boost with the CPU under a standard full load. Which is pretty decent. That's like overclocking your by 700mhz and adding four hyperthreads.
You would gain both single and multicore performance. Not as much, obviously, as upgrading to an 8th or 9th Gen Intel, or 3rd Gen Ryzen, but depending on how much you could get one for, it might be worth it since you wouldn't likely have to upgrade anything else so long as you have a capable enough motherboard to handle that CPU. What is the model of your motherboard?