Except once you have enought tabs Chrome starts putting multiple tabs in a single process. The cutoff seems to be somewhere in the low double digits. And at that point, you can in fact start running out of address space (not to be confused with running out of memory) without too much trouble, especially because system libraries on 64-bit OSes have a tendency to be mmap-happy, since they think they have lots of address space to work with.

64 bit address space significantly improves the effectiveness of ASLR, 64 bit tagged pointers can store way more data (7 byte strings, dunno if anyone actually does that, though).

I generally find Chrome likes to start falling over around the 3.5GB mark (as measured by chrome://memory) - larger images stop loading, pages stop rendering properly, and eventually it can end up crashing. Address space fragmentation I'm guessing.

This isn't helped much by Chrome's aggregate memory use being about 10x higher than other browsers. Takes a good 2-300 tabs in Opera to break 4GB, takes more like 20-30 in Chrome.