I can't select my dedicated GPU in the launch menu. I've tred forcing this option on to the game by selecting my GPU in NVIDIA control panel, but integrated graphics is still the only option. Can someone help me?

I'm wondering too. Even launching Mafia directly from the .exe isn't working with the dedicated GPU. It runs perfectly fine on my Intel HD 5500 though (except shadow flickering).

I sorted it, here's the guide I followed: http://steamcommunity.com/sharedfiles/filedetails/?id=1208975855
Download the dx8 to 9 converter and extract it in to the mafia folder. You should be able to select your dedicated graphics in the setup after.