Is overclocking safe with modern hardware?
Overclocking is safe, so long as you keep the processor's temperature and voltage within the manufacturer's specification. This is different for each CPU (or family of CPUs), so you will want to find the datasheet for your particular processor before changing anything.
Could an overclock and/or an associated overvoltage cause physical
damage to hardware, when talking about 20% increments?
Software can physically damage hardware, and you do happen to set the CPU's clock speed and voltage in software. Given a mild overclock, however, there will be no immediate physical damage to the processor - but it should be noted that high clock speeds, temperatures, and voltages are all known to accelerate transistor aging.
This isn't something you can completely mitigate - even running the processor at the default clock speed and voltage will wear down the transistors over time. This is all on a timescale measured in years, and the effects can be mitigated by compensating the CPU's speed and voltage over time.
In general, with modern hardware, what temperature ranges be safely
reached by components? I'm lucky that my CPU fan keeps the processor
40°C cold when strongly stressed (not overclocked). Can I expect a
physical damage at 90°C in a chipset? Or a video card?
Again, check with the manufacturer's rated specifications. I've seen some GPUs that are rated up to 100C, while also encountering CPUs that can only withstand up to 68C. Indeed, the chip will work past it's rated temperature/voltage, but the lifespan will be severely impacted. In general, the cooler, the better.
So, what does this all mean? For those users who want to achieve a 20% overclock, if you're aware of the risks and are comfortable modifying your processor's clock speed and voltage, go for it. While it's impossible to quantify how much you would reduce the processor's lifespan, it's probably safe to assume that you wouldn't shorten it past it's useable life in your system (especially if Moore's law keeps going).