timppu: Do I need to install both, or do they do the same thing?
Yes, you need both, just one won't offer full protection. Problem being whether the firmware update will be offered. Linux users have it easy, as Intel releases Linux firmware and there is a way to load it, though that makes the OS load it, not the BIOS, so in case of multiboot systems it needs to be applied to all OSs. On Windows they apparently don't, though there are programs that use the Linux code to apply it in the same way to Windows as well, but use at your own risk sort of thing. But maybe MS will release something similar, when Intel will get its act together.
timppu: Will PC vendors release new BIOS/UEFI updates for their old PCs anyway?
Doubt it. If anything, they may use it to spur people into buying new stuff even more, so it'll be "supported".
timppu: Last but not least: was it so that AMD CPUs are totally unaffected by the Meltdown vulnerability, so they don't need to care about either the OS nor the BIOS/UEFI updates?
As far as I know, yes in case of Meltdown. They are affected by Spectre though, everything is, including phones and little embedded CPUs in various other things. And Spectre is a tough one to fix.
timppu: Just try to act calmly online, not downloading and installing any freeware or pirate software from unknown sources. Hopefully that keeps me secure for now even with these vulnerabilities.
First attack vector is likely to be sites, any JavaScript, injected in ads, that sort of stuff. Since it's information disclosure, the vulnerability can't be used to write, delete or execute, data also needs to be sent, so it'll also escape notice while browsing but if running locally a proper firewall should do a decent job of stopping programs from connecting if you don't want them to.
timppu: Also I guess I will not buy any new PC for now, until this shit is sorted out somehow, or I know AMD CPUs are good to buy (even with the Spectre vulnerability).
Yeah, seems the smart thing to do, nothing currently or the market or released this year, at least. But with Intel apparently not even planning to fix hardware, in terms of Spectre at least, for at least the next few years, hmph...
timppu: One more thing. The articla warning about not installing recent Intel firmware updates said that "no you can't roll back either"... does it really mean you can't simply reflash the BIOS/UEFI with the earlier version? Why not, has that been prevented in some way?
Not sure, that confused me too.