monowall is indeed sweet. I'm running it on a
Soekris box as my router intalled to a 64meg cf card.
as you your windows replacement stuff i find that either gnome or kde both are very fucntional. while some command line bludgeoning may be required they both come with rather good administration tools.
As to file sharing NFS IS AWESOME. couple it with things like ssh and ftp acces and you have a very functional server admin package.
for those of you who need more than a command line:
http://fedoranews.org/mediawiki/index.php/..._Remote_Desktopi suggest fedora 4. its rather shiny.
Gaming is another story. supposedly cedega actually works and allow reasonalbe gaming experineces it is also a pay to play service. however, this does mean that is updated with great frequency.
as to debian versus KDE. its apples and oranges.KDE s a window manager meaking that it does all teh desktop stuff you expect from winodws. while debian is the acutal OS.
also try:
http://linux-newbie.sunsite.dk/index.html