Overview on distribution and version number with RPM (newbie)

[alex <alex99 sympatico ca>]

I'm a little bit confused with all  the version number and distributions
for RPMs.

the fields of a package are :

name-version(1)-version(2).architecture.rpm    distribution

Version (1) : does dependencies need to match version numbers. For
exemple is XFree86-modules-4.2.0 compatible with XFree86-4.1.0.

Version (2) : what is his meaning exactly ?

Architecture : Is there compatibility between architecture ? For example
I've download RedHat 7.2 for i386. Does this mean that I can install
only i386 package or that it is compatible with all i386 or higher
architecture and I can install any i386 or higher packages ? Is a
package build for i686 will be more efficient than one build for i386 ?

Distribution :
    1- Is there compatibility between distribution versions ? Will a
package build for RedHat 7.0 will work on RedHat 7.2 ? can we take
oldies back from RedHat 4.0 or 5.0 and make them work on RedHat 7.2 ?
    2- What are the main differences between distributions ? Is it the
philosophy, the directories structure... I mean from what I know until
now, each distribution have about the same kernel, most of them are
XFree86 compatible, they can run gnome KDE, Window maker and many other
graphical or textual similar applications, they usually use ext2 or ext3
file system... What are their differences then and most important why is
there so many different distributions ?
    3- Is there some compatibility between distribution. For example, I
have a Radeon graphic card and the XFree86-driver-radeon package is only
available for Polish and PLD. Is this mean tha I'm doomed and I have to
wait until someone built the driver for RedHat (because I have neither
time nor experience) or there is a simple way to make it work on RedHat.

thank and sorry for this huge newbie question.!