When looking at the black screen can you try to telnet or ssh into the machine from another computer? Is the computer running with black screen, or just crapped out (one indicator could be if the keyboard lights obey, and mouse is still powered)? Does ctrl-alt-delete respond by shutting down the computer?
How are you entering X, using a graphical login, or a startx command from the text mode?
"..the triumph of evil is for good men to do nothing."
VIDIA video driver RPMs for Fedora
:: see yum repo at livna.org