How many gigabytes of RAM does my Pi need?

There are a few use cases for which a Pi with 2 GB of RAM might be beneficial:

• Use of Video Conference mode - this application uses a lot of RAM
• Use of a local web browser (Web or Digital Signage mode) to display content that requires more RAM than usual (e.g. many open tabs, or complex client-side JavaScript)
• Use of dual displays at 4K resolutions, provided that you make manual alterations to /boot/config.txt to provide the GPU with more dedicated VRAM.

Otherwise, more than 1 GiB of RAM will confer no useful benefit.

I cannot envisage any scenario in which TLXOS would benefit from 4 or 8 GiB of RAM (increased use of bufcache might improve a dedicated PXE server's performance a bit, but not all that much), so you are just wasting money by buying those models.

There is no "gpu_mem_2048" directive, so by default our memory split is 256 MiB VRAM / 768 MiB main memory (gpu_mem_1024=256) for anything with at least one GiB of RAM. According to Pi documentation, the maximum possible value is 944, but allocating any more than 512 MiB of VRAM will not serve any useful purpose.