I'm still testing my treadmill script I explained above. The data (height of each vertex) can also be loaded into RAM and therefore don't needed to be loaded from HDD all the time. Only the textures need to be loaded at runtime from HDD (as I explained above too). Loading vertex height data into RAM would consum around 40 MB of memory with 20,000 tiles. In my world scale that would be an area of 576 km².

But as sivan mentioned. There's still the problem with unique entities on the terrain (buildings, vegetation, etc..) which increases the nexus.