Overview
The Atari 7800 was developed as a sequal to the 2600, evolving from a canned 10-bit system named the 3600. Although finished by 1984, it was shelved because the new management wanted to concentrate on home computers. (It took the success of the NES to cause them to release it in 1986).

Atari realized that people wanted to stick to their trusty old 2600's, so they build the 7800 [literally] with a 2600 inside it. This allowed users to play 2600 games, and then unleash the awesome power of the 7800's MARIA graphics processor with newer games. The 7800 also had one of the finest game libraries ever. It was also the very first games console that I ever had, and for that, I love it. However, it was considered a failure by many, having only a 10% market share in the USA. It did better outside the US, but came nowhere near to the success the 2600 had been.

Models
A couple of versions of the 7800 were released, there was only one difference, the first didn't have an expansion port, the second one did. The port did nothing, but the earlier models had a few compatablility problems with 2600 games. A prototype console with a transparent case, was also made, but never released :(.

Technical Specifications
Processor: 6502 (note: This is Atari's custom 6502 which can be halted to allow other devices to control the bus) running at 1.79Mhz (drops to 1.19Mhz when the TIA or RIOT chips are accessed for 2600 compatability)
RAM: 4K
ROM: built in 4K BIOS ROM, 32K Cartridge ROM space without bankswitching
Sound: TIA custom sound chip, same as the 2600
Graphics: MARIA custom graphics controller, 320x200 resolution with 256 colors.
I/O: Joystick and console switch IO handled byte 6532 RIOT and TIA
Ports: 2 Joystick ports, 1 Cartridge port, 1 expansion connector, Power in, RF output

Accessories
The 7800 had the usual assortment of joysitcks, and pads. Atari also developed a prototype Keybaord, that would allow your 7800 to act like an Atari 400/800 home computer. It would have been packaged with a BASIC compiler/interperter, and a Word Processing package. I assume that the elusive expansion port on the later models of the console was to plug in printers and other computer based accessories, unfortunaly we'll never know.