[CentOS] why centos 5 can not be installed in graphic mode on my pc

[Saturn]

hello all!
    I can not install centos 5 in graphic mode on my pc ,my VGA card is
XFX6800XT,when installtion started ,it tell me that my VGA card is 6600GT,it
detected wrong! And then installtion can not be run with normal display.
so I install in text mode ,after installtion , I can't start Xwindow! I only
could work with console.
I tried to install "NVdriver 9775",an error occured ,an installtion
terminated,I don't konw how to ....?
It took me a long time to download centos 5,it is really "fun"!
anyone help me ?
thanks
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