You need a display driver that is at least as recent as that included in the CUDA toolkit that you are installing. For example, Linux CUDA 5.5 requires a 319.37 or newer display driver, Linux CUDA 5.0 requires a 304.54 or newer display driver.
During the install of the toolkit, you will be prompted as to which components you want to install (driver, toolkit, samples). You can select no
when prompted for the driver, if you wish to keep your driver. There are getting started guides for each of the supported platforms (windows, linux, mac).