Pretty sure the browsers follow a similar approach to windows: If the memory is there it gets filled on the off chance that it shaves a few ms off somewhere because the needed item is already in memory.
I think wiskers is right though: It depends on the box. On my PC (2gigs) the browsers rarely make it to 1gig.
It's one thing for the OS to be making this sort of decision, since it has a global view of resource needs, a single application does not.
Actually, I don't even get the sense that chrome has an adequate view of its own resource needs. My wife often finds that her browser and computer is underresponsive because chrome is using so much memory for background tabs.
I think wiskers is right though: It depends on the box. On my PC (2gigs) the browsers rarely make it to 1gig.