I like to build my Debian PCs lean and mean.  He are some reasons why:-

Quicker to install:  obviously the less you install then the less you have to download and unpack.

More secure: a standard desktop build will put quite a lot of services onto your machine.  The last time I did a standard desktop installation I found things like apache and an ftp server installed.  All of these things introduce potential security vulnerabilities.

Less hard drive space and faster: Obviously installing less and takes less hard drive space, leaving you more to play with, and less services running means less processor power and memory used.

It isn't necessary to install more: Because of the Debian package management system, it simply isn't necessary to install more than you need.  For example if you install Redhat then even a minimum install will put quite a lot of stuff onto the machine in case an application that is installed later needs it. The Redhat package management tool (RPM) doesn't handle dependencies very well. I hate installing stuff on Redhat because you install an application using RPM but then find that it won't run because of some other file that the application can't find.  This simply doesn't happen with the Debian package management system (apt); when you install something with apt it checks that anything else needed is also installed.  If it isn't then it asks you permission to install that too.

Here is my method for building a Debian machine with KDE on it, provided that you have a broadband connection:-

Download a "minimum install" CD image and burn it.  Get it from http://www.debian.org/CD/netinst/
The last one I downloaded was about 110MB.

Boot from the CD, and setup your hard drive partitions and all of the other stuff, but at the appropriate point select "manual package selection".
This will take you to an application called "aptitude". Quit straight out of it by hitting the "Q" key.

Now you have a Debian minimum install.

Personally I don't like aptitude so I remove it by logging in and using this command:-
    "dpkg -P tasksel aptitude"

Next you need to get your apt sources correct
Check them by typing "cat /etc/apt/sources.list"
A reasonable sources.list will look something like this:-
  deb http://http.us.debian.org/debian stable main contrib non-free
  deb http://security.debian.org stable/updates main contrib non-free
If you are new to Debian then only use stable sources.

If your sources.list isn't reasonable then run "apt-setup" to fix the problem.

Get your minimum installation current by typing

    "apt-get update"
    "apt-get upgrade"

If you are building a server then you now have a good basis on which to build the server.  For example to make a web server you would now do:-
    "apt-get install apache2"

If you want to make a KDE desktop machine then do:-
    "apt-get install x-window-system kdm kde"
and sit back while apt goes and gets all of the stuff that it needs from the internet to do this (maybe 300MB)
so what are these things ?
    x-window-system is not actually a dependency on kde, because you could actually run kde and connect to an x windows server on another machine.  For a desktop I will guarantee that you don't want to do that, so you need an x windows server installed locally
kdm is the login screen that goes with kde, you don't have to use it but it is nicer than xdm
kde is the all of the kde applications and the window manager that sits on top of x windows

Once the install is finished you will need to do a reboot to get x windows sorted out, and that is it, you have a KDE desktop machine
In order to see all of the other stuff that you can install, like mozilla or openoffice, use the kpackage package manager to show you what is available

That is how I build PCs for my own use.  Other people will have different opinions.  Other people may also prefer Gnome over KDE but I know nothing about Gnome.