Amazon has this 4 DVD set for $10, a true bargain you shouldn't pass up. The Fly (1958) is a classic horror film, and alone it's easily worth the $10. Return of the Fly is a decent sequal, but not in the same league IMO as the original. The Curse of the Fly is a terrible film. The 4th DVD has all the special features, as the film DVD's have none.
FYI, the first film was in colour and the others were in black & white. Also, the 1980's Fly remakes, while fun to watch, are not as good as these originals IMO... the remakes are weaker on plot, and more about showing off special effects.
I own the previously released The Fly/Return of the Fly DVD, and this set has noticeably better remastered sound and video. Highly recommended.