So the major feeling about Macs for Gaming is that they become mostly interesting when it comes to make high end gaming PCs as far as the hardware is concerned. I assume MacPros then compete with Alienware products at that point, right ?
Macs aren't marketed as gaming rigs, so they don't "compete" with Alienware, but the Mac Pros are simply the fastest machines available, which is what a high-end gaming rig needs to be.
In a nutshell, it is the quality of Apple hardware components that makes Mac computers great high end gaming platforms ; not the Mac OS or whatsoever.
EXACTLY! Now that Macs are based on Intel hardware and can run Windows, it's really important to differentiate Mac "the machine" from Mac "the OS". The Mac Pro is a great gaming machine, but the Mac OS X is not a great gaming OS due to lack of game titles. Windows XP on a Mac Pro is a GREAT combo!
One thing that most people seem not to realize also is that, even though Apple's Boot Camp provides the necessary drivers to get Windows XP running, you can still update drivers as need be from the OEM supplier. For example... need a more current graphics driver for your nVidia? Get it from nVidia, just like you would for any other Windows computer. I had to upgrade my audio drivers to be able to play Eve Online, for example.
Obviously PCs remain the best as regards compatibility between most games and the Windows OS ; as well as regards their upgrading ability.
Nothing obvious about that at all. Today's Macs ARE PCs. The only difference is they have something called EFI rather than BIOS to boot the machine. Due to that, OS X's Boot Camp is needed to help get Windows installed, but once Windows is installed, it is NO DIFFERENT than any other PC from a compatibility standpoint. You can't do much to upgrade an iMac... they are basically a laptop that you can't take with you. The Mac Pro is a fully upgradable PC tower.
Now digressing a little bit: does Apple have factories of hardware components itself (for memory, motherboard, etc.) ? or are they produced by other hardware companies which dedicate part of their production to mac compatible stuff ?
Nope... Apple doesn't have their own factories. Their mainboards, for example, are made by Asustek. In fact, ASUS makes some of the best and most reliable mainboards for PCs, and I believe that choice by Apple to use them is much of the reason their Intel Macs are so good.