It will run any gpu with a pcie 16x slot, like the 3080. You probably will get a cpu bottleneck with a 3080. Please search for something cheaper like a 2070. to get less of a cpu bottleneck. (or even a 1060 if you do mostly gaming)
The few downsides of a second faster network card are that its bigger and it can be more expensive, but it can be used to upgrade an old pc for cheaper than replacing the motherboard. If you already have a motherboard, don't buy a new one, but if you still need to buy a motherboard, you can get a premium motherboard with fast networking.
There is another ...
If your stated options are the only ones available go for option 1.
My actual advice would be to try and get the exact same set of memory down to those CL15 timings or to completely replace your current RAM sticks for a 2x16gb set.
To answer your questions directly:
I Can add RAM of the same type but 16GBx1 in the 3rd slot? Will it work without problems?
could my the graphic card (NVIDIA GTX 570) be the limiting factor in the refresh rate?
It is absolutely this.
The GTX 570 is too old to have a DisplayPort that can handle 3840x2160 at 60hz.
Update your GPU, the CPU can survive a while longer.
You can easily tell if your CPU is the limiting factor by looking at the CPU usage in Task Manager. If the whole CPU ...
Maybe Threadripper would be better for server stuff when you need a lot of expantion. The ASUS Zenith II Extreme checks all of the boxes, but you need at least a 24core cpu with it (threadripper 3960X) and its a bit expensive (and eATX). Why the intel 11700 is not possible, please see my comment on the question. All motherboards that I know of run well with ...