The problem is how much
virtual memory you actually have available. You may have 2 GB RAM on the system, but the engine uses your virtual memory and if the amount available is lower than what is needed, you won't be getting anywhere.
For example, my desktop has 4 GB RAM, my total virtual is 6.50 GB and the available virtual is 2.43 GB. So in theory, I can build my levels successfully because the required cap (1.85 GB) fits within the 2.43 GB available. If my
available virtual was lower than 1.85 GB or less then I would most likely run into build issues. The key is knowing what you have to work with and what the software uses. If you have many applications running and the 2 GB RAM is used up, your virtual may be very minimal, in which case only a small segment room and a few entities you might be able to build.
A good util to check your system is
Speccy by Piriform. Once installed, you can click on the RAM section to see exactly what you have for physical memory and virtual.

There's no problem that can't be solved without applying a little scripting.