I have a 5th-generation Core i7 (5930K), 16 GB RAM, and a GeForce 1080 Ti. The game works smoothly on High graphics (around 50 FPS in cities). But, I would love to upgrade to Ultra. 
Do you think I can get away with only upgrading 1) the CPU and motherboard, or 2) only upgrading the GPU (e.g., to a 2080 RTX)?
2080Ti + 8700K @ 5GHz + 32GB RAM here I’m getting ~30fps with somewhere between high and ultra settings on 4K resolution. Looking at the developer mode FPS counter most of it has to due to GPU bottlenecking, You can enable it yourself in the settings and see where your bottleneck is.
MY suggestion is wait for the Nvidia Ampere (30XX series) release next month.
The gpu is extremely critical. An abundance of ram is good also, especially the fastest ram your system can accommodate. One other thing: an SSD is another good upgrade. I’m going to be buying a second and third NVME M.2 ssd drives in the near future. My Dell Precision Workstation has slots for a total of 5 NVME drives.
A time machine and jump ahead 6-18 months when optimizations to the game are done and/or faster gpu’s/cpu’s are available. Mostly game optimizations would help though.
your CPU will hold you back. Definitely upgrade your CPU first and see how it goes with 1080Ti. Probably 32GB Ram will help also, but you will have to buy DDR4 for your new CPU/MB combo.
thanks, this was good advice
Get the fastest CPU you can afford otherwise any GPU much greater than a 1080ti will bottlenecked by the CPU and hence you won’t see much benefit from a much more expensive card
I’ve got an i78700k slightly o’clocked (Hyperthreading Off gives me 5-8fps more)
1080ti
32 gig of Ram
With most things set to ultra and an LOD of 200 I’m getting between 30-50 fps (30 landing at an dense city airport )
The main thread (Core) runs at 100% and the other 5 run at around 80%
My 1080ti runs at around 70%
The bottleneck in my system is the main CPU thread
I would suspect if I got a top end i9 my GPU would quickly become the bottleneck)
Thanks. This is a newbie question – what do you use to determine “main thread usage” vs other-thread usage? A CPU meter averages across all threads, and on a modern processor, the average usage is really low.