I just copied all of my windows true type fonts into the /usr/X11/lib/fonts/TTF/ directory and executed fc-cache as root. You can also reboot if that didn't work since fc-cache is run on startup.
In Gnome, you then have to set Gnome to use the new fonts in your Desktop Preferences->Fonts.
In KDE, you might have to go into KDE Control Center as root and in the System Administration section->Font Installer to update the fonts KDE uses.
You could also update Gnome by installing http://www.dropline.net/gnome/
which has some (not all) the Microsoft True Type Fonts. Plus you get i686 optimized versions of a whole crap load of software.