I am not familiar with how Fedora implements its 3rd party driver installations (such as the proprietary nVidia driver), so I can only give some general advice based on the error messages.

From your errors....

nvidia-settings-319.37-30.fc18.x86_64 and xorg-x11-drv-nvidia-1:331.20-7.fc20.x86_64

The issue seems to be you are installing a package designed and built for Fedora 18 (with nVidia driver version 319.37) onto a Fedora 20 system (with nVidia driver version 331.20). So various things in the package you are trying to install are already installed and of a more recent version - which may or may not work together nicely, so you are prevented from installing them as things will most likely break in a non-trivial way.

Is the CUDA installer giving you the option ~not~ to replace the already installed nVidia driver?

From their FAQ at: https://developer.nvidia.com/cuda-downloads
Q: Will the installer replace the driver currently installed on my system?
A: The installer will provide an option to install the included driver, and if selected, it will replace the driver currently on your system.

According to that you should be being asked whether or not to install the driver version included with the CUDA package or not - if you get that option, tell it not to replace the driver and it ~should~ just install the CUDA 5.5 SDK without touching the driver.

Hope that helps some.