if you are using propietary driver, you must first uninstall radeon driver (if aplicable), and blacklist it. Then disable KMS and then install propietary driver. As far as I know, ATI drivers are not compatible with KMS (Kernem Mode Setting, this means modeset=0, or nomodeset).
It's very important to forget a little bit about the Windows world..Just forget it and keep in mind that you are in linux world, nothing here is automatic or taken for granted. If you are to install a driver of any device, you need to unsinstall the driver used at the moment.