There's "radeon" already which doesn't provide hardware 3d accelaration. For that your options are either MESA (check their site) or propriteary ATI driver called fglrx (check ATI site if your card is supported). There is probably a binary package for fglrx on Mepis, you should use that. Second option is to obtain the installer from ATI's site and install it "manually", which may prove difficult. ATI's drivers are known for being stubborn and not work nicely, so...be warned
