You have to close the X server. If you boot up to a gui, this is a little tricky, but not impossible. However, it just tells you to exit X so that if there are problems, you won't be completely buggered.
You can easily leave X open and perform the install from the console, and if you run into problems, you can pass the kernels some neat little option that I'd be glad to help you with. Don't worry though, nvidia is extremely reliable, especially with redhat systems.
I doubt you will have any problems.