Can I run it without a CPU-fan, with just a heatsink?
Not likely.... a 1.2ghz can cook up some heat. I wouldn't dream of taking the big-ass hsf off my 1.2ghz T-Bird.
You could use a Zalman cooler, or some other low-noise solution, though.
Also what is the fastest Intel or AMD cpu that can be run without a fan?
Not very fast. I haven't seen a PC faster than a couple hundred mhz without an active fan somewhere. (now granted, some later designs up in the 400-500mhz range used a large heat-sink on a cpu and routed air from the power supply fan over it...) But, even then, I've not seen one over 500mhz or so.
I heard that the AmigaOne G4 has a CPUfan, that is not cool, why?
My A1200 060 50Mhz doesn`t need a fan.
You just typed the answer, yourself. a 50 mhz chip with ~2.2 million transistors (68060) vs 700+ mhz chip with over 6 million transistors (g3/g4).
To run a chip that is that much more complex, you need more power. (You're not going to run a modern computer on a 50-100watt A1200 power supply, either!) You add more power, you get more heat. More heat means you need a better way to dissipate it. Hence, larger fins, more fans.
