Sorry to hear that as it's never nice to loose a pet.

You're probably better off letting your gf mention a new cat herself as everyone is different. I had to have one the very next week when our lil one died, but some people need time to mourn. Let her decide.
Oh and for those interested, the reason cats don't usually show any sign of having been run over, is because their body is extremely flexible. They will have internal injuries but their bones might not be damaged in any way...