I would like to build a server for hosting services and DNS to become an IAP in the futur. What I wonder now is why a server is so expensive ? It has the same CPU speed as a PC and no impressive feature. And also, how much Oracle and IBM sell their db ? Why people buy them while MySQL is open sourse ? Should I use Unix or Novell, Linux for a server ? Seemingly people love comercial open sourse product like Unix, Redhat (Google use redhat to host their search engine).
Sorry to be stupid, could you help me for these small concerns ?

One very big point when considering servers is stability. I could run a server on my desktop, but chances are that after a (long) while the memory, cpu, or northbridge will make a mistake and something will grind to a halt. While there's never really a way to 'guarantee' stability, server providers do the next best thing by only using the very best components in a proven combination to provide the smallest chance of hardware problems: it is this that you pay for.

One great example of this is the Intel Xeon processor, which is a standard server CPU. When you look at the specs, you can find some comparable with a Pentium 4 HT. However, if you actually stick them both in machines, you find that the 'rated' specifications on the Xeon are far below what they are capable of - they are essentially underrated in order to increase stability. Take my desktop machine: I run dual 2.4GHz Xeons, but overclocked to 2.97GHz! You can't get that kind of (~25%) performance increase from standard desktop cpus, because the server CPUs are just much more stable. I would imagine similar effects are found when considering every other component in a server: quality of the registered memory, quality of the fans and power supply (very important), etc.

Of course, people run servers on desktop-style machines, and I guess it just depends on how important what it is serving is. If you are looking to build a reasonably light server, I would certainly recommend basing it around a pair of 2.4GHz Intel Xeons (not overclocked) with an Asus PC-DL deluxe motherboard.

I know that doesn't answer many (or any) of your questions, but I'm sure it's relevant somehow!

The admins of another vBulletin site I read might disagree with that, given the recent demise of their Dual 3.06 Xeon HP DL380 after less than a year in operation,
http://www.boards.ie/vbulletin/announcement.php?f=9
http://boards.ie/vbulletin/showthread.php?t=190111

I'd guess people buy servers for the compact rackmount case (pay per U in co-lo), efficient cooling, vendor support, and the sense of security (real or imagined) knowing they have a machine designed to be hammered 24/7.

Databases: I'd guess companies buy commercial when the benefit of vendor support, tools, and tax rebate outweighs the cost of self-support. Also the abovementioned sense of security.

Where did you hear about Google and Red Hat? It may be that Red Hat is actually paying Google to use their distro so that this fact can become public and enhance Red Hat's reputation.