DDR5 ram currently doesn't exist outside of a lab. The X470 motherboard only supports DDR4. Again what ram the motherboard supports has no bearing on what vram the video card can have. For example my current system uses a Z370 motherboard with DDR4 3200 and a GTX 1080 that has GDDR5X.
GPU RAM/VRAM doesnt have anything to do with the sytem ram.
Yes you can, and GDDR5X is somewhat faster and thank nvidia to milion versions of their GPU's since they had to re use the VRAM chips cause they were overstock with it.
Only it comes down when you up the quality and ram becomes the limit (Eg you have 8GB's and whole 8GBs is loaded with games stuff, vice versa).
PCIE, thats where you connect the GPU.
In some instances , for reference your is 3.0 on Motherboard and GPU.
GPU can work on PCIE 3.0, 2,X.1.X and etc.
Expect some very antique 775 socket or some prebuilt motherboards which dont accept new GPU's from 1000 series and up from nvidia cause of compability.