It's not normal for Windows to reserve 2 GB memory for itself, at least not in a 64-bit version of Windows 10. What I think is happening is that your integrated graphics chip is reserving that 2 GB for itself, because that's what the Radeon Vega chips seem to do these days, perhaps after a recent driver update. (Or perhaps I've been missing it for a while.)
You can change the memory allocation for the chip in BIOS, if you want; this is completely fine to do and shouldn't break anything. In your position, I'd assign 512 MB and leave it at that. But if you don't have anything else open while you play Sims 3, then 6 GB RAM for Windows and the game is enough, and you technically don't need to do anything. Just don't open other apps while playing, because even a couple of browser windows can take a significant amount of memory.
By the way, your Nvidia driver is new enough that I think the Control Panel should have a global fps limiter built in. This works in fullscreen and windowed modes. Right-click on the desktop, select the Nvidia Control Panel, then Manage 3D settings, then under the Global tab, scroll down to Max Frame Rate, set it to On, choose 60, and click Apply.
