The reason that you won't find those things on the system requirements page is that Concrete5 is simply code that runs on top on a machine set up to be a web server. It's not an operating system, and it won't refuse to install or run if you don't meet a hardware spec.

I'll try to answer your questions though:
- Concrete5 is best run on a Linux/Unix server with Apache and MySQL (LAMP). I believe it runs fine on a Windows machine with Apache (but I've never done this myself). Don't run it on IIS. It runs comfortably on a mac using MAMP, but this is just a development environment really.
- Hard disk space - I would put aside 50meg for Concrete5 (and a theme or two), and then you'll need to add space for any media you upload. I'd allocate, say, 150meg for a small website for some breathing room. This says nothing about what it takes to run your operating system, etc.
- RAM - If you are just running this on a home machine, this shouldn't be a concern if your machine isn't ancient and covered in dust. Servers don't give you all the RAM they have anyway, they have a maximum for each page load. Each PHP process shouldn't need much more than 64meg. The only time I've run out of memory with Concrete5 is when I've done some custom programming that was lacking optimisation. 128meg is perhaps a good value.
- Processor Speed - Any modern processor should be fine. I'd suggest that hard disk speed may end up being more of a factor than processor speed these days. A machine that has a slower processor will run a CMS like Concrete5 slower, but it will still run.

If you are running Concrete5 on a proper LAMP server (ie a company offering hosting), you shouldn't have to worry about hardware specs. If you are just running it in an office or at home, any decent machine should run it fine.

Hope those are the kind of details you were looking for.
-Ryan