Familiarity: -If you are using Redhat, then CentOS is the closest you will get and all the support and updates you get for RedHat will work for CentOS. Any Linux distro will work well, but sticking to an RPM based distro is going to be most familiar territory for you, so another alternative may be Fedora.
Graphical Desktop: -These distros will run on 256MB if you choose a lightweight environment..i.e. don't run Compiz/KDE or Gnome...use Enlightenment or LXDE. So if you are happy to migrate from RPM, then Bodhi will work very well...
Performance: -The fastest graphical performance will be from ultra-lightweight distros...like TinyCore/Slitaz. PuppyLinux will also work well. Gentoo and Vector Linux will also optimise the performance.
Run Headless: -If you wish to run the client computers as a headless (i.e. control then over the network, no monitors/graphics cards or keyboards) then configure the distro to use your hardware most effectively, (so get rid of desktop environment, and modules that connect to things like printers, scanners if these are not going to be connected.
Run Clustered: - If you wish to maximise the productivity from your multiple client computers you may wish to conder running them clustered. I am not an expert, but the PelicanHPC distro will (theoretically) make your low powered machines act like one powerful one. Whether this can be used practically for your purposes is down to you.
All the above will cost Nothing
Heterogeneous environment: - If the OS on one or more of your clients is going to be Windows the I guess XP is your only answer that will work you. While it is not something I would generally recommend, it has the following advantages...ability to run software that you can not run on Linux, allows other computers on the network (even Linux ones) to access these Window only utilities over a remote desktop, and allows you your test your servers when accessed through a windows machine.
Mac OSX will not work with the specifications you have given
Good luck