Your problem is likely from trying to use rpm with Slackware.
All you need to do is download the latest linux drivers from the nvidia site (in the tar.gz or tgz format, NOT RPM!) and follow the instructions. I believe all you have to do is `tar -xzf` each file and then switch to the new directory and run `make` as root.
After installing for the kernel driver and the GLX driver, you will need to recompile the kernel. You should do this anyway to get max performance out of your system, but if you don't feel comfortable doing that, go to kernel.org
for instructions. If you're still running the default Slackware 8.1 kernel, try to get rid of some hardware support you don't need at the same time to decrease the size of the kernel (increasing performance dramatically in sime cases).
Then, use utiel's XF86Config file as a sample for what you need. The changes required are minimal.
In the future, don't use RPMs with Slackware. It can be done, but I've had nothing but problems with it. If you have more Slackware-specific questions, check out the forums at linuxpackages
for access to some real Slackware gurus.