I couldn't find any information about the system architecture of an Arduino (CPU, RAM, buses, the protocols it uses, etc.).
How about here: http://arduino.cc/en/Main/Hardware
I guess you didn't understand the question. The difference between hardware and architecture is a bit like this: your pc has a CPU, RAM memory, probably a graphics card, etc. Hardware is exactly these parts, architecture is how they work together (read: communicate; work as a whole).
Or get the datasheet from atmel.com, has block diagrams of internals and everything.
I've looked into those, but there's not a single bit of explanation why they made some choices, or, again, how the architecture is made up.
The "system" components are all inside the AVR chip; Atmel picked out the particular combination.
As such, there isn't much "systems design" TO an arduino, beyond picking those central AVRs. This is one of the reasons that Arduino is criticized in some circles; it's not really much more than a breakout PCB for an AVR chip...
Ah, I see, I'll contact Atmel in that case and hope they'll be able to give some detailed information. Thanks for the help!