I think that the only accurate prediction relative to memory (processing memory and storage) is that people will want as much as they can reasonably afford to buy. There are a lot of different definitions of what "inexpensive" is, but everyone knows what a bigger number is.
If 500GB costs the same as 750GB, the manufacturer will sell the 750 because it has a bigger number. And software folks will find ways to fill it up.
"120GB are about all you need to service the average PC today."
And 640K is all the memory the PC will need, too.
Seriously, I know the next version of Microsoft Office stores data in the cloud by default and my son likes that (he's a student and most of what he does needs to be sent to professors) but I don't and the companies I work with like it even less.
So most of our data will be kept locally even if not directly on the PC.
But, hey, I've probably got more than 120MB of programs in my Program Files directory.
And then there's the size of the operating system itself...
Yes, but I think this could be a little misleading. Consumption is easily going to outpace storage, simply because a lot of what goes across the wire or the ether is never stored, and not meant to be.
The most obvious examples are any form of streaming media, or for that matter, broadcast radio and TV that preceded Internet streaming. All those "bits" of information sent to millions or billions of households were never stored, and still aren't now with streaming media.
But also consider just filling out forms on some "cloud server." Like, your bank, or your lawn service. The packets flowing back and forth between client and server mostly bounce the same information back and forth, only storing at the server a few entries the client sends in.
Just think about regular old face to face conversations. Imagine if they were quantified as bytes. I suppose some of them are stored in our brains, but most of them simply drift off to never-never land.