the skill level of the administrator/installer and the intended purpose of the machine are all
factors in deciding upon a distribution.
Gentoo keeping around the sources and running Gnome 2.30.2 is 21.2 GB This is a huge
number compared to most package managed distro's Much of this is the sources in
/usr/portage/distfiles/* (4.1 GB)
Compiling software on old slow hardware can be quite time-consuming as well.
However...it's quite easy to tailor make your own system with Gentoo's package management and
the judicious use of portage. How to make your own SystemRescueCD for instance.
Create your own custom ebuilds and even your own binpkg host if you
wish or a cross-platform toolchain for embedded development.
If your skillset is more limited but still you would like to try linux; Ubuntu is a good choice for first time users of linux systems that "just want it to work". Even so it is not bug free. A recent kernel update added a new grub boot option on a system I built and without copying the needed "nomce" at the end of the kernel line. Result Lucid Lynx unusable for the user. On a working new system built for linux. I don't know why grub chokes without
the "nomce" flag but I'll file a bug as soon as the user sets up his Ubuntu One Care account so
they can hopefully address it.
Got introduced recently to NerdVittles and PBX in a flash from some acquaintances at lugot.org (which seems to be down atm SMH)
I see Asterisk has an ebuild in portage as well.