The first Atari home computers were the model 400 and 800, introduced in 1979. The main difference between the machines is that the 800 has a monitor jack that the 400 doesn't have, as well as having more memory and an improved keyboard (comp.sys.atari.8bit FAQ). In 1983, the Atari 400 sold for $100, the Atari 800 sold for $300, and the Atari 1200XL sold for $500 (Consumer Reports, p.474).
Processor: The Atari 8 bit series uses the 6502 microprocessor, which is capable of addressing 64k of RAM.
RAM: The Atari 400 came with 16k of RAM, the Atari 800 came with 48k of RAM, the Atari 600XL came with 16k of RAM, and all other XL series machines came with 64k or RAM.
Keyboard: The 400 used a less desirable membrane keyboard, but all other models include a fairly good typewriter style keyboard. The Atari keyboard includes most all standard ASCII characters, the only ones missing are characters 96, 123, 125, and 126 (`, {, }, ~). The keyboard does include function keys and standard control and shift keys, so software can easily use other keystrokes to replace those missing. Arrow keys for cursor movement are obtained using the control key in combination with other keys on the right hand end of the keyboard, near the enter.
Video: The Atari has what many consider one of the better video systems, capable of 40x24 text resolution as well as good graphic resolution. The Atari 400 and 600XL only include a radio frequency output for connection to a TV set. All other models include a composite monitor jack as well. The 800 includes the chroma and luma signals needed for a separate video (S-video) monitor as well, but the chroma was unfortunately dropped on the XL series. The use of a separate video monitor produces sharper color than standard composite. See the Super Video XL modification under the "Resources" section below for more information on the missing signal and how to add it an XL. (comp.sys.atari.8bit FAQ).
Storage: All 8 bit Atari computers have a serial bus with a 13 pin port that allows both drives and serial/parallel devices (printers and modems) to be attached. Most of these devices are fairly advanced containing their own processor. Originally Atari used drives that had custom boards making it impossible to change drive mechanisms but later drives used standard drive mechanisms allowing replacements and 3.5 inch drives to be used. Other companies sold drives for the Atari as well (Atari 8 mail list).