Atari started out as a game maker. Nolen Bushnell, Atari's founder, invented pong and created Atari to bring it to market in the early 1970s. By the end of the decade they decided to release a computer system to compete with Apple and the rest of the "home" computer market. In 1979, the Atari 400 and Atari 800 were born to fulfill this promise. Since they were Atari products, they were supremely strong graphical performers.
The Atari 800 in my collection is a 48K unit with a tape drive and an 810 disk drive (pictured below). I have several Atari cartridges and most of the original manuals for the machine as well.
Mike Newhall was nice enough to write in with some corrections (I swear I'm going to set up a comment system someday!)
"Processor Motorola 6502"
The 6502 was a Rockwell design, licensed to various parties (including Commodore). But I don't believe Motorola ever made them. They had a competing series, the 6303/6305/6309, that were used for example in the Radio Shack Color Computer.
"Speed 1 MHz"
The speed was exactly half the TV color clock signal, which for NTSC works out to 1.79 MHz, almost twice as fast as the C64.
"RAM Up to 48K"
True for the 'official' specs of the 800. I have one with 288K using 3rd party mods. Atari later made the 800XL w/64K, the 130XE with 128K, and there were 3rd party mods for up to a megabyte of RAM.
"Video Up to 320x192x16"
The 320x192 mode was really "one and a half" colors - the background was one color while the foreground was a different brightness of the same hue. Special "GTIA" modes added to later 800's allowed up to 16 shades of one hue, 16 hues of one shade, or 9 independent colors in 80x192. The highest-rez mode with normal-shaped pixels and normal color behavior was 160x192x4. So there was no true "x16" mode.
But the graphics hardware was extremely flexible - in the vertical dimention. One could change the color palette on every scan line, and even change the number of displayed scan lines, so it was easy to get all 256 colors on screen at once, in resolutions of NNNx200 to NNNx224 or even more. You were just limited to how many colors you could get on a single horizontal line.
All great info! Thanks! Appropriate corrections have been made in the table at the top of the page.