After asking you for the amount of available memory on your graphics card, the installation program asks you to specify the "clockship" of the card. With the exception of particular hardware, choose "No clockship", its detection normally being automatic.
Finally, to finish, you can execute "X -probeonly" which attempts to initialise your card to see if has been recognised correctly. If you have any doubts about the choices made previously, do not do this test which can at worst completely block the installation procedure and force you to start again.