I have 8GB RAM in my system, and Windows 7 64-bit.
For me, 8GB is useful if I have virtual machines running, or if I have a lot of large programs running at once (for example, Crysis, Firefox with about 50 tabs, Photoshop CS4).
Having 8GB RAM allows me to do a lot, and at the same time I can still have page file disabled.
That makes my system very responsive when switching between programs, and it also means the Operating System has a lot of memory to use for superfetch.
So when I launch frequently used programs, it doesn't take long at all for them to load.
Photoshop CS4, for example, would usually takes a while to load. But on my system, it takes about 1-2 seconds.
As far as I know, today there's no application that can eat up all 8gb of RAM.
But you might not necessarily be using only one application.
For users that basically use the computer to browse facebook and email, then 1-2GB RAM is enough.
For users that like to play some games and do more demanding tasks, 4GB is usually enough
For users like me, 8GB can be beneficial