*NO SPOILERS*
There were questions earlier in the thread about the available DVDs. Here's a list --
Babylon 5: The Gathering*
Babylon 5: The Gathering / In the Beginning*
Babylon 5: Season 1
Babylon 5: Season 2
Babylon 5: Season 3
Babylon 5: Season 4
Babylon 5: Season 5
Babylon 5: The Movies*
Babylon 5: Legend of the Rangers
Babylon 5: The Lost Tales
Crusade: Season 1
* - B5 has always been a testing ground for Warner Bros. The initial release of B5 was the pilot movie (The Gathering) and the more well-received (and later) movie In the Beginning, as a flipper. One movie per side. It sold enough to justify putting the first season out on DVD.
At the time they put out season 1, they had no intentions of releasing anything else, and the bonus features are lacking. But season 1 sold enough to justify the decision to release the remaining four seasons, which had more bonus content, include cast commentary on one or two episodes. (One, I think)
The Gathering has been released three times. First, on that flipper disc with In the Beginning. Then as part of The Movies box set, and later as its own one-disc release. Warner Bros. used it as a test (again) when it released a dozen or so pilot episodes of its various series at around 5 dollars. If one bought the pilot disc, there was a coupon that could be used to take 5 dollars (or something like that) off the price of the first season box set of that particular series.
It's rare that The Movies box set is under 40 bucks. But it doesn't contain all the movies ... It is missing Legend of the Rangers (which came out around the same time), and naturally doesn't have The Lost Tales, which came out 2-3 years ago, as a one-disc thingy.
Your first time through, unless you've already shelled out for the movies, I would suggest watching the five seasons in the order they are on the discs. "Midnight on the Firing Line" is the first episode of the series proper and gives you a re-introduction to the characters and settings.
Some of the movies take place during the series, some before, some after. Don't let yourself get bogged down in that. There are at least 3 different viewing orders that I've seen posted. Ignore them. There's no need to complicate things like that your first time through. Go with the five seasons the first time through, then watch the movies.
The only caveat there is, if you're going to watch the spin-off Crusade, watch it after you've seen the movie A Call to Arms, which serves as a bridge between the two series and a sort of pilot.