Atari 400 and 800
The 400 and 800 models were the very first Atari Home Computers and featured a full keyboard, although this was an awkward membrane one on the 400. Released in November 1979, both machines used the ANTIC and GTIA chips (earlier models used a CTIA chip) to give 256 colours (this was almost unrivalled at the time) with a display of up to 320 x 192 through 14 graphics and text modes. For the techies amongst you, the machines also used raster interrupts and could display 4 sprites.
Graphics 9 mode allowed the machine to display gray-scale digitized photographs, which were very impressive at the time.
All of the Atari 8-bits came with the Atari BASIC programming language, and from experience I can tell you that this wasn't as easy to program as many other versions around at the time, but could produce really good results when you got the hang of it. Atari had intended to use Microsoft BASIC, but couldn't fit it onto the 8kb ROM cartridge.
Originally, the smaller 400 was supposed to have only 4kb RAM, but the falling price of memory meant that 8kb was fitted on both models.
Despite its dodgy keyboard, Atari sold twice as many 400's as 800's. The fact that it was cheaper probably helped, but it also looks more like a gaming computer than it's bigger brother, which may have been more appealing to youngsters during the early 80s. There was also little support for the extra cartridge slot on the 800 by developers.
What were the main differences between the first two Atari computers? Well, the 800 featured a proper keyboard for a start, making it look more like an actual computer. It also had an extra cartridge slot, and memory slots that were expandable up to 48kb.
However, like the 400 it used the MOS 6502 CPU, along with same combination of ANTIC and GTIA chips to provide graphics, and the POKEY chip to handle sound.
Interestingly, other rival computers of the time such as the Commodore PET and Apple II also used the same CPU processor.