1) Yep, yum. GPG keys assure the packages you download via yum have not become corrupt. You will want to read THIS
page. There are instructions and an example of a good yum config file (located in /etc/yum.conf The base repository configurations are located in /etc/yum.conf.d as separate text files. That site also explains how to install the GPG keys for the repositories you choose.
2) The control center for Gnome is called the Configuration Editor. There are several other config utilities, and they can be found under "System Tools" in your main menu. KDE...I've no idea, sorry.
3) Congratulations on getting your ATI drivers set up. This is not always a walk in the park. Unfortunately I do not know how to sort out your artifacting problems.
You should _definitely_ go through the entire FAQ at the link above. It is the Unofficial Fedora FAQ, and contains a wealth of tips.