Stupid, stupid me.
When I installed red hat originally I didn't install the kernel source because I figured "when the hell am I ever going to need to recompile the kernel, I'm just writing a little bit of C code and doing everyday usage stuff."
If only I'd known.
I popped in the red hat disks and installed the rpm(s) for the kernel source, then the updater brought it up to my current kernel version, and that solved my problem.
Then the nvidia installer was able to find the headers and compile itself something useful. It worked, I was happy.
Now if only I could get a driver for my audigy ls