Brands

You can use different brands of stuff inside a computer, no problem at all.

There is no concept of "a CD unit for a Dell *insert model number here*" or similar concepts for pretty much all hardware. The only exception is the memory and some Dell power supplies - older computers may not like certain types of memory and Dell power supplies have been covered before. Oh yeah, Dell motherboards are unique as well. No you cannot put them in another case.

Brands are of no consequence, it's no different to having a JVC television and plugging a Panasonic video player and a Bush DVD player into it, then connecting a Sky Digibox made by Grundig. People are happy to mix and match on any other equipment that can be plugged into something else but seem insistent on having a like-for-like replacement inside the computer like it makes a difference.

The car goes along the road regardless of whether you put Dunlop tyres on it or Mitchelin tyres on it. The engine really doesn't give a toss. Most computers really don't care that the hard drive is made by Western Digital or Seagate - long as it can boot from it, it makes no difference.