Something I've wondered is how do people design these ridiculously detailed models of motherboards/fans/other components? One example is QinX's model of the 92mm Noctua. How did he get the curvature exact? And for motherboards, how does one model all of the circuitry?
I don't know about the curvature, I can only do extrusions and stuff like that, but the most of the circuitry on motherboard models is just a texture on the board, they don't paint all the traces on there
For component placement on the boards, you can often use the manuals, the drawings in there are to scale in most cases.
Still, no idea how people can possibly make these curved fanblades.
About the original question:
If you just want to see how you can fit components inside a box and for first sketches of the design, SketchUp (as the name indicates) is a very good tool. It's extremely easy to use, you'll find a lot of help online and good part libraries are available as well, as Aibo said. Making up small parts is quite fast as well, so you can test what kind of designs you'd like quite quickly. Using it for actually
producing your case can pose a challenge, though. To my knowledge, SketchUp doesn't have built-in tools for the creation of technical drawings. It is possible with plug-ins, but may require a bit more work than other tools do.
That's where Solidworks or Autodesk Inventor come in. Those programs are equipped with dedicated tools for sheet metal modelling and bending, press features and of course technical drawings. If you want a metalworking shop to do the manual labour for you, you'll need to make technical drawings that include tolerances and details of more complicated parts of your case, because the drawings are what a manufacturing contract is based upon, even if it's just for one unit.
Both Solidworks and Inventor are available as free educational versions, but that means that they explicitly are not allowed to be used for commercial use, so if you want to sell your case, you'll generally have to buy a full version to stay within their license agreements.
IIRC, NCASE use both SketchUP and Solidworks in that they first draw up all the stuff in SketchUp and then transfer it over to Solidworks for production, and if you are just starting with 3D modelling, I would recommend you do the same. Start with SketchUp, get a few of your components online and see whether your design ideas would actually work.