The great thing about computers versus cars is that computers, more or less, are standardized. Meaning you can build from the inside out, not the outside in. For example, processors are built to fit in a socket. Once you've selected your processor, you find a motherboard with that socket. Then you find a case that fits the motherboard, which is also neat because larger cases can fit smaller motherboards (they use standardized mounting holes). Add-on cards like video cards fit in the same slot too, the only consideration you have to worry about is when you get into small cases, but if you're not building a small-as-possible computer, this isn't something you have to worry about.
So what it really boils down to now is what you're going to use this computer for. Gaming? Media creation? Just something to surf the web?