We know it's not exactly a fair comparison as Grand Theft Auto IV came out on PC two years ago, but we've caught a bunch of screens of GTA IV and Mafia 2 on our PC gaming rig to compare the graphics. All screens have been taken in 1440x900, and as you can see GTA IV sports impressive textures, while the lighting and the lack of jaggies stands out in Mafia 2.
Anyway, have a look at the comparison shots: