There's no way in hell that Thor makes $400 million domestic. Spider-Man barely did, and he's one of the most recognizable pop culture characters. Very few movies make $400 million domestic. Thor will not be one of them.
However, I think Thor has an edge over Green Lantern. Both have fanciful characters and feature otherworldy realms. However, Thor is going to have a lot of press tying it into Marvel's growing world of films. GL won't be related to the Batman film world in any way. Also, GL is just a bit goofier due to him having a magic ring. Ring vs hammer? I'll go with the hammer. Lastly, GL is going to have numerous CGI characters. I'll go with guys in costume over CGI creations any day. And I think the public will too.
On to this threads' original question. Thor's box-office and reception is important to the Avengers' reception. However, Avengers will be shooting and potentially have principal photography wrapped by the time Thor comes out in theaters. It's not like Avengers will be canned if Thor doesn't live up to hopes and hype.
I think Cap will be even more interesting in terms of acceptance. People are overplaying how the name "America" will play overseas. It's a period piece set during the most popular (in terms of interest) war. Yes, it'll fit the superhero mold, but it also has a toe in the historic realm. This could be very helpful in setting it apart from the norm. And Cap is not going to be leader of the Avengers in the film. Fury is the leader. Iron Man/Tony Stark will likely have a far more important role in the film and likely be considered the leader. Maybe they'll be vying for leadership. I wouldn't expect Cap to automatically be the leader.
I think Thor and Captain America will each bring in about $200-250 million, but likely be in the low 200s domestically. They'll have openings of around $70+. I don't think either will near $100 million opening week, unless they have some incredible trailers and promotions. Marvel did well with promoting Iron Man, so maybe they'll be able to do wonders with Thor and Cap, too.
BTW, Iron Man, Cap, and Thor have always been my top 3 superheroes. I've read their comics for 30+ years. I'm hoping Thor and Cap join IM in popularity with the public. I'm also a skeptic, though. And realize they both have their uphill battles.