Spider-Man and Wolverine were in the Avengers for a very short time. Generally, the X-men characters that are considered "real avengers" (as in they spent significant time with the team) are Beast, Scarlet Witch, and Quicksilver.
Spider-Man's been an official Avenger twice. The first time, he was just a reserve member (but was briefly called into active duty shortly after, largely just cleaning up the mess caused by the enemy the main team was fighting), but he was later made a regular when the New Avengers team was formed (and he's been a member ever since). Wolverine's only been an Avenger since he joined the New Avengers. But, yeah, Scarlet Witch, Quicksilver, and Beast served the longest as Avengers of all the mutants (with Justice and Firestar being the only other mutants. Well...technically Namor as well).
The classic Avengers lineup is leader Captain America, Iron Man, Ant-Man, the Wasp, Thor, and Hulk when he's not too pissed off. This is probably the basic lineup they'd use for the movie, but possibly with Hulk removed because he never sticks around much due to being totally nuts.
The initial Avengers line-up was Hulk, Thor, Wasp, Ant-Man, and Iron Man. Hulk left after one or two adventures and Captain America was retroactively made a founder. From everything that's been said, the Thor, Iron Man, Incredible Hulk, and Captain America movies will all lead into the Avengers movie (no word on how the Ant-Man movie's coming along or if he'll be in the Avengers movie). And, within those movies, we've seen, or will see, Black Widow and James Rhodes (who, as Iron Man and War Machine, was briefly an Avenger twice, both times with the West Coast team), with Don Cheadle (playing James Rhodes as of Iron Man 2) confirmed to also be in the Avengers movie in some capacity. Whether Scarlet Johannson (playing Black Widow) will be in the Avengers movie is anyone's guess.
The team has been through dozens of members and incarnations, but aside from the core members above, the main Avengers are generally considered to be: Hawkeye, Quicksilver, Scarlet Witch, Vision, Beast, and Ms. Marvel. Other popular members are Black Widow, Wonderman, Luke Cage, and Spider-Woman.
Beast has never really considered a "main" Avenger (his tenure was fairly brief, and aside from the occasional help in major crises, he's never returned to being a member). And Spider-Woman's tenure has been too short for her to be a popular member (she's as popular a member as Dr. Strange, really). I'd throw Hercules, Black Knight, and She-Hulk into the "popular" category, though. And there are a number of others who are popular, but not so much as Avengers (Moon Knight, Captain Marvel, Thing, Mr. Fantastic, Invisible Woman,Namor, and the original Human Torch).
Since they're going with the Ultimates depiction of the team, we'll probably see Nick Fury/Samuel L. Jackson as their commander and the team working for SHIELD.
Yeah, we'll probably see a merging of the Avengers/Ultimates stories (maybe Fury brings them together, but they fight Loki in their first battle).
