It will install the latest version available to you, depending on your
choice of debian repositories.
Use synaptic, or search your apt-cache to find the name of the latest
version and install a newer version.
I think stable has 1.8.2 currently.
If you need to add new repositories, you can. then apt-get update, and
search for the 1.8.4 version of Ruby.
Debian always prefers stability of package selection over cutting
edge. If you always want the latest, I would suggest Fedora, but that
way lies the infamous “rpm hell”, and why I haven’t touched Red Hat
since the late nineties.
Apt-Pinning for Beginners has a way to keep
stable, yet install packages from testing when you want to.
SuSE is RPM based, but yast handles dependencies pretty well.
For Suse the current stable version of 10.0 lets you install 1.8.2,
but suse rpm’s are available here for 1.8.4:
SuSE Yast:
http://download.opensuse.org/distribution/SL-OSS-factory/inst-source/
SuSE Yum:
http://download.opensuse.org/distribution/SL-OSS-factory/inst-source/suse/
They show version 1.8.4.16 from April 29th as being the latest for 586:
http://mirrors.kernel.org/opensuse/distribution/SL-OSS-factory/inst-source/suse/i586/ruby-1.8.4-16.i586.rpm
-Sean