I would go the Ubuntu route in the client computers and maybe Ubuntu Server or Red Hat Enterprise on the server. The advantage of Ubuntu is the community. You can find help for most common problems on the Web. As a matter of fact you should choose a distribution which has an active community. Most distros do, but some like Ubuntu are growing everyday. The major difference between Ubuntu and Fedora/RedHat is the package management. Ubuntu being a Debian variant uses DEB packages and Fedora/ Redhat uses RPM. Most software can be found in both package types. If you are willing to try different OSes you could try PC-BSD. It is a FreeBSD variant (UNIX Variant not Linux). It uses KDE as the default Desktop Enviroment (GNOME,XFCE are the other 2 most common desktop enviroments). It might be a little more complicated and you may have issues with drivers, but there is an added benefit with security and a longer history and more experience community (Although not as active as Linux comunities). Anyway, play a bit and enjoy the ride!

"The simplest solutions are the most difficult to find" --Me