You can enable multisampling in your app or outside it. Outside is usually done via environment variables or XF86Config options, and overrides the multisampling that the app requests. Windowed and full screen should work with either method.
Doing it from the app:
I haven’t tried it on ATI+Linux, but I have with NV+Linux. You can do it with either GLUT or GLX (glew doesn’t create contexts, so it won’t do it for you). With GLUT, pass GLUT_MULTISAMPLE to glutInitDisplayMode. I’m not sure how it chooses the number of samples to request.
With GLX, you need GLX 1.4 or GLX_ARB_multisample (see the ARB_multisample spec). Assuming the ARB extension, add GLX_SAMPLE_BUFFERS_ARB=1 and GLX_SAMPLES_ARB=n to the glXChooseVisual call.
Once you have a multisampled context, glEnable(GL_MULTISAMPLE) should be all you need.