To gnome or kde is a matter of taste, you can install both if you want and try them out.
Apps that are made for gnome require gnome libraries to run, but should run in kde. Similarly, kde apps need kde libraries installed but will run in gnome. Your choice is really one of desktop taste rather than what applications you want.
You can continue to use thunderbird. For browsing, firefox is good. As for video, mplayer and xine are popular, xine particularly for dvds. For music, xmms is a popular player with many good plugins available to perform some dandy tricks. These are probably shipped in your distro in which case you may choose them at install time.
Happy computing, and don't be shy in the forums
Sharing problems is a linux experience.