apple_noob :
Ok thanks so much velocityg4. I'm thinking the iMac is the best. I realize an old Mac Pro would be better if I upgraded it but its very old and is going to stop receiving OS X updates in a year. Plus a flashed 1080ti costs a fortune.. thanks again
You're welcome. For long term support and ease the iMac is best.
I don't know where you are getting that 1 year number from for the Mac Pro. Apple doesn't publicly release plans like that. It could be next years OS X it could be five years from now. Even when it isn't supported, there will likely be workarounds. Unless they switch to ARM CPUs.
Just for clarification. You don't need a flashed 1080 Ti. A flashed card just provides the boot screen. Which can be useful for boot issues. As long as you keep the old card for diagnostics, OS X installer, &c. That shouldn't be an issue.
All you need to do is install the drivers for a regular 1080 Ti
Founders Edition. Non FE will likely work but FE cards are built to spec. So, there is less to worry about for compatibility. You get a black screen at boot. As soon as OS X gets to the point in the boot sequence where it detects hardware and loads drivers the card turns on. You also need a power cable adapter.