As phantom said, try looking at my post. It has the steps that worked for me when I installed a NVidia card. Tell us if it works out for you. Unfortunaly, I've heard nothing but bad things about the rpm drivers for Red Hat. The tars are much more reliable. There is nothing particularly more difficult about it, just a few more commands you need to type. Refer to my post (linked in phantom's post) for step by step instuctions. I hope it works out for you.
BTW, if the drivers messed up your system, which is sounds they did, you should be able to set the driver back to "vesa" in the XF86Config-4 file and you might be able to get back to some sort of a X desktop. At a command promt type the following to do this:
su
(you need root access to alter the file)
pico /etc/XF86Config-4
Change Driver "nvidia" or Driver "nv" to Driver "vesa"
Save and exit (ctrl +x, y, return)
startx
See if that helps. It worked once for me, another time it seemed my X was damaged beyond repair, so the good 'ol install disks came out once again...