A tab should be completely and utterly destroyed when refreshed. It doesn't matter how much memory it used, refreshing it or navigating away from that page should (ignoring optimizations, caching, etc. etc. etc.) be like you were never there.
Just look at the hundreds of complaints here: http://neosmart.net/blog/2008/firefox-3-is-still-a-memory-ho...
Proof? This was the case with Firefox 2 but now it just seems like this is part of their reputation and is not necessarily grounded in fact.
I'll vote for process-per-tab as the single best feature of Chrome.
Lately, I'm close to leaving Safari for Chrome because it's faster, uses less memory and crashes less (in fact, it's never crashed so far, vs 100's of times total probably with Safari.) Unfortunately, Chrome is a little uglier, and has certain less user conveniences. So it's not an easy jump to make.
I also get more crashes with Chrome than I used to get with Firefox, but maybe I'm just opening more tabs.
If your free memory isn't close to 0, your OS is wasting RAM or you don't have much anything running.