OK, this would be a Slackware-specific issue. You should probably use the tarballs instead of the RPM's. RPM's record a number of dependencies on other RPM's. These dependency checks include such basic packages as the one used to install /bin/sh. Because Slackware does not user RPM's to install it's components (such as /bin/sh and X), RPM will fail the dependency checks for the nVidia drivers.
You have two options: you can install the drivers from a tarball instead of using the RPM's, or you can ignore RPM's failed dependency checks. I recommend using the tarballs--if there is something missing, it will probably be easier to dig in and find out what the problem is with a tarball than with a RPM. However, if you have checked all the dependencies and feel confident you have everything, you can tell RPM to ignore failed dependencies. I believe the option for this is "--no-deps", but check the man page.