markaldric

i wouldn't bother upgrading this yet either.
 
GPU first, then maybe add more RAM if you need it. 
intel CPUs of that same socket are not compatible with older chipsets. (eg: you can't buy an 8800k and put it in a board with a z270 chipset or lower.)
even if you have a BIOS update.

If you're just playing at 1080p60 I wouldn't even bother upgrading. Save up for next year when we'll see what Intel brings to the table.

Assassins Creed Odyssey, just like the previous AC titles, is quite taxing on the CPU. It really likes more threads.
If the performance you get isn't satisfactory, I would first upgrade the CPU, but sadly, in some cases, that would require the motherboard to be changed too.
 
Replacing only the GPU (or the GPU first) would yield mixed results - some games would profit, but most new titles wouldn't.
Simply put, the 6500K would bottleneck a stronger GPU on 1080p in those games. It even does so now in some games.
 
There's three ways to go with such a system.
1. a higher resolution monitor + a stronger GPU + an i7 6700K or 7700K
2. a new platform (CPU+MBO, eventually RAM) + a stronger GPU [and then later a new monitor]
3. upgrading only the CPU to an i7 6700K/7700K
 
So, you choose, an expensive (used) i7 6700K/7700K that fits your system if you don't plan on upgrading the GPU, or a new platform with a CPU that has more than 4 cores.

Used prices on 1080s are decent, can't really go wrong.