You could try hitting CTRL+ALT+F1(or F2, or whatever, up to F6), then edit your /etc/X11/XF86Config file to remove the offending resolution. It might also be /etc/X11/XF86Config-4, depending on how Redhat does things. Ideally, you'd also want to shutdown X windows. I don't know how to do it in Redhat, but usually you run the init script for you login manager(gdm,kdm,whatever) with the stop command.
In Gentoo, for example(It might be similar in Redhat), it would be something like:
In the worst case scenario, you'll just have to rebuild your XF86Config file. There might be a tool to do so in /usr/X11R6/bin called xf86config that can get the job done for you. I'm pretty unsure about most of this though, as it's been a long time since I've used Redhat, and I don't quite remember how all their stuff is laid out.