DDR5 64Gb is already available, and if you're running the 64 bit versions of Windows 10 Professional or Enterprise editions, they can address 512 Gb RAM. The limit is defined by what the OS can support, but motherboards can add their own restrictions. The processors themselves can support a lot more.
But I don't think that realtime ray tracing of entire frames is going to become viable in the next 5 years, and possibly never. You can render a scene now using ray tracing, but it takes a long time compared with the frame rates we expect in a game.