I chose the Silverstone GD02S enclosure because of the nice quality aluminium finish and the compact dimensions, only 360 mm deep. The motherboard comes from Asus in the form of the M3A78-EM for HD capability and the energy efficient 780G chipset.
An Athlon X2 e-series with 2 x 2GB DDR2 completes the package.

The first drive is a solid state 32 GB for the Debian operating system in amd64 testing flavour. The root partition is set at 20GB. A directory hd1 in the home partition is mapped to a big hard drive, making sure it spins down when not used.

Power comes from a Corsair cx400 with 80+ efficiency and very quiet. One of the internal USB headers is connected to a bluetooth stick for remote control with a mobile.

If you are old school like me when installing a PC, i.e. taking a really old Debian netinstall, installing a minimal system and upgrading the distribution, forget it with a more recent motherboard. The ethernet needs a driver, a lenny disk will work.

Starting with a clean basic 64-bit install, and after setting /etc/apt/sources.list, here we go:

# KDE
apt-get install xorg xserver-xorg-core numlockx kdm kde synaptic ssh ntpdate

# downloading things
apt-get install ktorrent limewire-basic gftp kget tftp rar unrar bchunk

#bluetooth
apt-get install kdebluetooth bluetooth obexftp obexpushd anyremote

# browser
apt-get install iceweasel sun-java6-plugin
(the repo 64-bit flash plugin is not yet that good, better download the alpha library from Macromedia and copy it to /usr/lib/mozilla/plugins/)

# sip
apt-get install twinkle

# multimedia
apt-get install w64codecs libdvdnav4 libdvdread3 debian-multimedia-keyring alsa-utils
apt-get install kaffeine kplayer acidrip k3b libdvdcss2 dvb-utils lame amarok kradio

# OpenOffice
apt-get openoffice.org


Now that we have most of the software, we can add some convenience.
My HTPC and monitor is disconnected from the mains when not used to ensure maximum energy saving. To enable a remote power on, set the appopriate flag in the bios. This makes the HTPC turn on when power comes back.
The automatic login can be configured in the menu: settings > system administration > login manager > convenience.
A multimedia keyboard can be configured in the menu: control center > regional accesibility > input actions.
The sudo command is handy for executing a root action without logging in as such. It works really swift when you configure it to not prompt for the password. Add this line in visudo: my_account ALL=(ALL) NOPASSWD:ALL.