Does Fedora Core support the x1000 series graphics cards "out of the box." I know if you want decent performance you will have to download drivers from ATI's site, but will Fedora Core be able to install with my x1800 xl?
no, it won't. But you should be able to install the closed source drivers by livna during installation since anaconda now supports addition repos. But I haven't tried that.
At least you should be able to run 2D-Mode out-of-the-box by setting driver "vesa" in /etc/X11/xorg.conf. Hardware accelleration won't be available doing that.