Earlier quoted context omitted.
You just look at the hardware and read reviews. There isn't really a sliding scale for PCs, that's the awesome thing about manufacturers that let you customize. Doing a lot of gaming or modelling? Better graphics card. Running shitloads of processes? Better ram. Better processor. Extra HDD, or SSD, or optical drive? Etc.
Nothing about driver compatibility :( it's a long and painful process to figure out if the computer you want will even work with the software you want to use. Of course I'm sure it's better if you use the blessed OS and version pair, but I've never found a reason to use windows before....
Having one manufacturer and fewer models usually means longer and more consistent product support.