The physical design of the Atari 800 was radical, but not for reasons related to the aspects you highlight.
As others have commented, using multiple boards for a system was pretty standard at the time. S-100 systems for example were based on a backplane, with system features implemented on multiple boards connected to that. (The Atari 400 and 800 were designed using a Cromemco S-100 system — I don’t know whether that had any direct impact though.) The Atari 800 has three main boards: the “motherboard” (containing POKEY, PIA, the memory decoder, and all the system connectors), the CPU board (containing the 6502, CTIA or GTIA, and ANTIC), and the power supply board. User-accessible boards (initially sold as cartridges) provide the ROM (“system personality”) and RAM (not firmware though — what we’d consider firmware nowadays was provided by the external devices themselves).
The main drivers behind the design appear to have been:
- user-friendliness, which led to building an enclosed system with well-protected user-accessible parts, such that the user could upgrade the system without being exposed to such things as the power-supply;
- compliance with FCC requirements, in particular because of the inclusion of an RF modulator — this is the reason why the system is so heavily shielded.
Both of these were radical departures from the other personal computers of the time; the first in particular had far-reaching consequences. Instead of opening up the whole system to access expansion cards, Atari 800 users could open the first flap to connect cartridges containing software, the second flap to change their system’s ROM or RAM, and anything else was connected to the joystick or SIO ports. To keep all this safe for users, the flaps had interlocks which ensured that the system was powered down when the flaps were opened; unless the users went out of their way to circumvent them (well, it wasn’t that hard really, but it couldn’t happen by accident), there was no risk of connecting or disconnecting a cartridge while the system was powered up. To keep all this easy to use, the designers had to come up with some rather novel ideas: the SIO bus in particular was far ahead of its time, with device-supplied drivers loaded automatically (the descendant of SIO is USB, amazingly enough).
Coming back to the multi-board design, it does have some nice features, which might explain why it was kept at least for the initial system. Having a separate power supply board means that the main board and power supply board can be designed separately, by experts in the respective fields — these different kinds of boards require different kinds of expertise to design well. It also means that the main board can be bread-board designed without too much regard for the power supply; Goldberg and Vendel’s Business is Fun shows that bread-boards were used extensively. Keeping the CPU, TIA and ANTIC on a separate board might have been relevant because that whole section of the computers took a while to settle; the same book shows that the designers were still contemplating different CPUs late in the system’s development.
One very nice benefit of using different boards like this, is that the CPU and RAM boards could be re-used as-is for the Atari 400 (which was re-designed, or perhaps just re-specified and designed, in a hurry not long before launch). The Atari 400 shares the same design, with a separate power supply board, main board (which includes the ROMs), and separate CPU and RAM boards (neither of which are supposed to be user-serviceable in the 400).
Another nice benefit is that the base memory configuration could easily be changed to take advantage of price variations in the market. The Atari 800 was supposed to ship with 8K of RAM (hence its name — the Atari 400 was planned with 4K), but as RAM prices fell, its base configuration improved progressively until it ended up shipping as standard with 48K a few years later. This would be possible with socketed memory chips on a big motherboard, but that approach is a little more restrictive. The Atari RAM boards are in some ways like early SIMMs, and allowed Atari to increase chip density without changing the motherboard (the boards were available in 8K and 16K variants, with the same number of chips).
(The design wasn’t without its flaws of course. The initial cartridge design used for ROM and RAM made for poor airflow and created thermal problems, so the metal shielding around the cartridges was removed in later models, and the second flap’s plastic latches were replaced by screws. For completeness’ sake it’s worth noting that expansion wasn’t limited to the external ports entirely; the last RAM slot could be used for other kinds of expansion boards.)
The Atari Museum’s page on the Atari 800, the Wikipedia entry, and the above-mentioned Atari Inc.: Business is Fun all give further background on the design of these systems.