If still have some control over your system then you could try reinstalling Gnome. You should be able to kill the Xserver with Ctrl-Alt-Backspace, or switch to virtual console by doing a Ctrl-Alt-F1. Alt-F7 will get you back to X.
Most likely the problem lies within XF86Config-4 and your graphics card. Check out the "VideoRam" setting and also the "HWCursor" setting.
Also check out these dirs/manpages:
man 5 XF86Config-4
Also go to http://www.xfree86.org/
and browse through all documentation related to your graphics card.
Hope this helps.