9400M shared memory always 1/8 of total system?

Looking at MacBooks, iMacs and Mac minis with the NVIDIA GeForce 9400M graphics, which share memory with the main system.

I notice on all 2GB machines the graphics has 256MB of SDRAM and the machines with 4GB the graphics has 512MB. Does this mean if I bought a base model mac mini, with 1GB memory and 128MB SDRAM, increased it to 4GB of memory, I would have 512MB of shared video SDRAM?

 

But if I bought the basic Mac mini with 1GB and therefore 128MB video and upgraded to 4GB I would get at least the same 256MB of video SDRAM as if I had bought the top spec Mac mini, with the larger hard drive I don't need?

 

Thank you, just saved myself £100! Only need the 120GB HDD but wanted the maximum memory.