I saw this post on Neogaf where someone said that the OS itself will take 800mb cause some random dude leaked it or something.
I don't think that the OS won't take up any ram tbh because this really isn't the gamecube days. The OS will most likely be doing things in the background or have the ram reserved for things such as if you want to browse the web.
So I do think that if we are being realistic, we should assume it will have 3.2gb reserved for games or less.
And yes, as like how one of the dudes mentioned, it depends on what kind of ram is it?
Really, there is only 2 to choose from in the LPDDR4. 25GB/s or 50GB/s. If we compare it to xbox one's memory bandwidth, the x1 has 68.3GB/s. But the x1 has ESRAM that can help with the memory bandwidth which is 102GB/s. And ps4 has 176.0 GB/s.
So it is pretty interesting to compare, that is for sure. If it really is Parker without compromises, 50GB/s + Pascal would be interesting to see. Specially since Nvidia always had lower Memory Bandwidth (and other things) than AMD but still managed to out perform them.