Domestically, Avatar has already broken the record for biggest second weekend of all time (it grossed $75.6mil beating TDK's $75.2mil) and is estimated to absolutely murder two records this weekend; biggest 3rd weekend (currently held by Spidey 3 at $45mil) as well as biggest weekend in January (Cloverfield's $40.1mil) where the estimate is currently sitting around $68mil. Its $25million gross on New Year's Day was another record.
The key here isn't the numbers (New Moon did $142, then $70, TDK did $158, then $75) but it's actually the dropoffs and the timing.
Most blockbusters experience about a 50% dropoff or more during their second week, but not Avatar. It's staying the course so far, and with no major released scheduled any time soon (lol Book of Eli) it should be able to keep that course. A 77/75/68/60/55/50/45 run could go far it beating TDK and Titanic, especially if it wins Best Picture at either the Golden Globes or Academy Awards to give it a boost. Factor in the grosses during the weekdays and Avatar has serious potential to take out TDK and Titanic. If Cameron can pass TDK and hold the #1 and #2 spots in domestic box office grossing history then he will surely be the undeniable king of film... even more insane is doing it with two non-franchise films!
Anyways, Titanic will be tough to beat because it pretty much had no dropoffs ever and was #1 for 15 weeks in a row or something (which is probably Titanic's most impressive record of all.)
http://en.wikipedia.org/wiki/List_of_1997_box_office_number-one_films_in_the_United_States
http://en.wikipedia.org/wiki/Box_office_number-one_films_of_1998_(USA)
So yeah. Titanic will be tough to beat domestically, but with the higher price of movie tickets these days it might just be possible.
Avatar is doing even better domestically, so it should easily crack $1billion and get to the #2 all time spot, but Titanic's distance from the other billion grossers (about $800million more) is just too great to overcome. Note how Avatar is already #30 all time worldwide after 17 days of being released, and also note how an average not-even-blockbuster modern film like Ice Age 3 is ranked #13 all time.
http://www.imdb.com/boxoffice/alltimegross?region=world-wide
So my predictions are;
3rd all time domestic gross: LOCK
2nd all time worldwide gross: LOCK
2nd all time domestic gross: just over 50%
1st all time domestic gross: about 25%
1st all time worldwide gross: maybe just under 10%