If you want something powerful and portable, check out Lenovo's X series. I am a 99 percent full-time Linux user and Lenovo consistently impresses me with their product's durability and longevity.
One good thing about ThinkPads is that most of them do not have their RAM soldered to their motherboards, so it is easy to buy your own RAM on NewEgg and then augment or replace the machine's original memory.
in general, thinkpads have good support for linux. i've had various x series laptops over the years and they've been very solid (more so than my partner's macs, where i need to keep replacing fans and power cables).
for desktops, just build what you want. as long as you avoid strange or very new hardware it generally just works.