If it's what I'm thinking of (looks sort of like your display is being digitally scrambled), then I'm fairly sure it's a refresh problem. Your video drivers are probably okay; it's most likely your monitor configuration that needs to be tweaked.
First (you asked
), see if you can find the manual that came with your monitor. If you don't have it, you may be able to search for the model number on Google
and turn up some specs. The things you want to know are "Horizontal sync" and "Vertical refresh", each of which should be a range such as "30 to 75 kilohertz" or "50 to 100 hertz", respectively.
Then, using a GUI tool to do the configuration or by editing your /etc/X11/XF86Config-4 file directly, enter those values in the "Monitor" section of the configuration. If you're using a GUI config tool, it ought to just ask you what the rates are; if you want to edit the config file directly, the lines will look something like this:
Identifier "Some monitor name"
Restart X, and the settings should take effect.
As for the ATI drivers, the .rpm file can be installed using a GUI installer (such as rpmdrake, if you're using Mandrake), or, in my opinion an easier way, just through the command-line using the 'rpm' tool. Open up a terminal window and type:
rpm -Uvh /path/to/the/ATIdriver.rpm
If all goes well, it'll do some stuff and when it's done you'll have the driver installed. You may want to check out any READMEs or other documentation provided by ATI, since they may have more explicit instructions on how to install it, and whether you need to do any other configuration.