Chances are the disk is just going bad. In fact, if it's a floppy you're booting off of, and you've booted Linux quite a bit, I'd almost guarantee it. While high quality floppies might last a thousand runs or more, most of the ones you see nowadays (because they're being phased out anyway) aren't good for more than a hundred or so uses without "refreshing" the data on them. For most people, it's no big deal, but when you're using it to boot your OS, it's a major headache.
Okay, to your problem. Your best solution is to use your install disks, provided Redhat has a rescue option on them. (I started with redhat, but moved on and haven't run an official redhat in a while.) When you drop that cd into the drive and boot, you should get something that says options, and you hit a key, and it takes you to the options, one of which should be a rescue system. It may (for some distros) be on the second or even thrid disk as well. (I believe 5.2 used the second disk, but it's been five years+.)
As long as you get a 'boot:' prompt, and you are running the standard kernel that Redhat came with, you're home free. Try typing "root=/dev/hda2 noinitrd ro" at the prompt, where hda2 is your second partition on your first hard drive. hdb3 would be the third partiion on the second hard drive, and so on. Another option is to use another distro (knoppix is a good choice), and chroot over to your redhat system and then make another boot disk. If you're a newbie, forget that last option, it will probably confuse you more than solve your problem. On the other hand, keep in mind that a Linux system is never "dead" no matter the symptoms (aside from the HD being in pieces, of course.).
That said, you'll likely have trouble with your boot with a new card. It's not that big of a deal in reality, but it will be a headache to someone without experience, without anybody to turn to. Luckily, you have LQ.
Anyway, you'll need to get the the command prompt (the dos-like thingy), log in as 'root', and then run "xf86cfg -textmode" This will drop you into an easy to use, if primitive looking, setup program for your Xwindows setup.
Select the nv driver for your card, and you'll be alright. It may not have your card listed, but all NVidia cards are backwards compatible (well, all the newer ones), so even if the driver isn't top of the line, it should still work. You won't get 3D worth a damn, but it will work. Later, once you're up and running, just pop over to Nvidia, follow their links (it's a bit buried, but not too hard to find), and install the proper linux drivers for your card. (I like NVid's cards, but I wish they were a little more cooperative with Linux...That said, I own two of them.)