this is a very simple problem (although i dont know how it arised)
you need to setup your X configureation file....
please post the contents of your /etc/X11/XF86Config file.
or u can try edit it yourself....
lookk for the screen resolution part, is your graphics card and monitor capable of displaying that resolution ? if not, lower it, "800x600" WILL work (although looks pretty bad)
also, have a look at the refresh rate, is you monitor capable of that refresh rate.. if not change it (look in your monitor instruction manual)
look for the driver section... what does it say ?
since you didnt tell me what distro you are using, im not sure what it should say,
NOW..... look for the modules load section....
there should be a Load "glx" and unless you installed the nividia.com driver
but please give more information,
What distro ?
have you installed the drivers from ati.com ? or are u using the generic drivers..