The Atari 2600 did come before the 400/800 series, but it was named the Atari VCS and came out in 1977. It was renamed to 2600 in 1982.
The 400/800 Series came out in 1979, and was very comparable and in many aspects superior to the Commodore 64 from the get go (and the 64 didn't come out until 1982)
Atari was wise enough to make the 400/800 capable of playing VCS carts. The initial versions of the 400/800 were discontinued in 82, but various successors with the same software capabilities were released throughout the 80s, but lacked the 4 joystick ports (and to my understanding, that really ruined things for M.U.L.E. on the Atari, which was released with 4 ports in mind and didn't have an alternative) There may be some sort of expansion for at least some of the newer PCs in the 400/800 series that allowed 4 joysticks, I'm not well versed enough in Atari to know for sure.
I did score an Atari 400 with ram expansion to 48k when I was 12 or 13. I had several carts with it, but didn't have a disk drive, and I sold the computer to an Atari collector through a local BBS, since my world revolved around the Commodore and I didn't follow the Atari crowd. (I think I made $35 or so, that was big money to me back then.) In hind sight, wish I had kept it and learned more about Atari.
M.U.L.E. was released only on Floppy Disk and therefore, was not planned to function on the VCS/2600.
Want to see Atari's timeline?
http://www.landley.net/history/mirror/atari/museum/Atari-Timeline.html