For desktops Coreboot list support for two current AMD CPU motherboards here:
http://www.coreboot.org/Supported_Motherboards
However, you're going to be left without 3D support unless you want to
introduce non-free code in the form of an nVidia GPU VGABIOS. Both of the
write ups seem to indicate you need some hardware tools, so trying to find
someone friendly at your local Hackspace or Hacklab would be well advised.
For new laptops that don't explicitly guarantee support for free software
then look for a PC shop who, like Novatech in the UK, does their own brand
laptops 'naked' (i.e. no O/S). You'll need to speak to a techie rather than
a sales assistant but you will be able to find out about WiFi (and changing
it), TXT etc.
Although IMO you're better buying second hand locally. Then you've tested
with a Trisquel Live CD/USB stick so you know it works, it's better for the
planet and you're not keeping that war in Africa over one of electronics' raw
materials going. Free software being more performant than proprietary most
people don't need the latest hardware.
Which leads to the point that other than for web browsing, editing digital
media and games most computing needs can be met by the command line and
either Emacs or VIM. If you take the time to learn these things then you'll
end up like me, with a 5 year old desktop that rarely gets above 10% CPU
usage and stave off an upgrade until actual hardware failure or accident
occurs.
