You have a couple of options:
At the grub boot prompt (i.e. when booting) - hit any key to stop the countdown. Then select the current kernel (should be only one if this is a fresh install) and hit "e". Three more lines will show up (like below):
kernel /vmlinuz-2.6.11-1.1369_FC4 ro root=LABEL=/ rhgb quiet
Select the second one (kernel...) and hit "e" again. Append the line with the number 3 (i.e. ...rhgb quite 3 ) and hit return. Now hit "b" and the system should boot into run level 3.
Alternatively, once the system has already booted, hit "CTL+ALT+F1" to get to a terminal. Log in as "root" and type:
This will also bring down the x-server, and put your system into run level 3
Run your update (nvidia?). Once this is complete, you can either reboot, or simply type :
from the same terminal, and X should restart.