I don't specifically know FC, but I expect that it is similar to several other distros that use a desktop environment.
If you login via a GUI screen, then there is probably an option from that screen to specify which DE you want. Poke around there before you log on to find how to choose the DE. If you have installed KDE, it will probably be on the selection list. Just select it and log on into a shiny new KDE environment.
I'm not sure you can blame GNOME for your video quality. It usually looks pretty sharp for most people. There is probably something else wrong, and you may see the same results in KDE. Also, sometimes the fonts that are initially installed are not the best and you may get better looking results if you get some more.
If you do get the same looking results in KDE, experiment with your xorg settings, especially those that deal with color and resolution. Be sure that before you do that you have a backup of your original xorg.conf file and that you know how to sign on in single user mode to restore the original.