Linux will give you more support for recent hardware and more windows-like applications if you intend to use it as a desktop OS, depending on the distribution.
However, if you only need this machine to be a standalone file/web/database/mail/whatever server, the BSDs are much more compact and very easy to set up and maintain, imo.
You can get a bootable CD image of the minimal FreeBSD installation at freebsd.org, which will give you the OS, mail, the ports list, and network support, basically. If you have a static IP or DHCP connection you can install whatever else you want from their FTP site directly through the OS. Their port maintenance system is very well thought out and very well maintained. You get a list of makefiles and documentation for each application, basically, and if you want something that's listed all you have to do is change to its directory under the ports tree and run an install command, which will result in the OS going out to a list of FTP sites, downloading the required sources, compiling them, and installing them automatically.
The manual for the OS is here...
http://www.freebsd.org/doc/en_US.ISO...ook/index.html
Specifically read the section on ports and the pkg_add/pkg_delete commands for getting what you need installed.