The Wall is an album any self respecting Floyd fan must know, and good in its own right, but there are other better ones just as indispensable to serious fans. Dark Side definitely qualifies, and is probably the best of the lot, but some might argue for Wish You Were Here, which is every bit as indispensable if only for "Shine on You Crazy Diamond I-IX" (which is the bulk of the album.) Meddle and Animals are rather vital as well, though I much prefer the former, and not just for the 20+ minute B side that consists solely of "Echoes." If you are really serious about Pink Floyd, Piper at the Gates of Dawn is an obligatory album; it is the first one, and the only one with extensive Syd Barrett content (I want to say Syd is on a song from Atom Heart Mother, but do not quote me there.) Still more obscure but still very good, IMHO, is The Final Cut (be warned; it is also probably the worst of Pink Floyd albums for sudden, extreme and frequent volume changes.)

In particular, in terms of band history:

Piper is, once again, both heavily Syd Barrett and essentially the only one featuring the former founder whose influence defined much of the bands character for the rest of its existence.

Wish You Were Here is, in a very real sense, a rock biography of Syd, who eerily visited the studio during its recording, so overweight, bald and more dissociated than ever that no one in the band recognized him at first.

The Wall is basically "the Roger Waters rock bio" and Rogers dictatorial attitude reached such infamous proportions on it that it was the bands death knell. In fact, Roger forced Rick Wright out of the band during recording of the The Wall (Wikipedia claims he threatened to "pull the plug" on the recording unless Wright left the band.) As a result, it was the last album to include all members (except for Syd, who David Gilmour once said in an interview was dropped en route to a gig when "someone, I think it was Roger, said, 'Let's just not pick up Syd tonight.'")

The Final Cut has many songs David Gilmour refused to put on The Wall, and is in many ways the same album as The Wall with a global rather than personal perspective (I sometimes refer to it as "We Are the Wall," but perhaps that reference is dated.) I still consider it the final album, and it was the last album with Roger Waters. I once heard Roger say in an interview that the band had already agreed to break up while recording The Wall, but their label called him up and reminded him they had signed a contract for two albums, not one, and thus The Final Cut was made, cementing the rift between Pink Floyds primary lyricist and its primary musician (according to moondog, Roger Waters was such a poor base player that David Gilmour plays the driving baseline in "One of These Days" at the start of Meddle.)

Some people love A Momentary Lapse of Reason and The Division Bell, but I am not among them (though Momentary Lapse ultimately grew on me and is an OK album, which is to say, there are five songs on it I genuinely like now rather than just three.)

Cannot help you with Foo Fighters, so I will not try. If you need more information, and more knowledgeable authoritative information, I highly recommend NBing/emailing moondog. Not only can he tell you far more than I, he has every Pink Floyd album ever made, including Ummagumma, which is cool to have if only for its iconic cover.

And I'm currently listening to a commercial for Pawn Stars while I wait for Lock 'n Load to return.