Supermodel's card requirements are pretty fluid and depend on how high a resolution you plan to run at. Absolute rock bottom is defined as supporting OpenGL 2.0: for ATI cards look at the table
here and for Nvidia cards
the tables here will do the trick. FPS may not be ideal towards the lower end of that range unless you stick with 640x480 resolution though.
In general, an Nvidia 8800GT or an ATI X2000 or better should run Supermodel well without being the bottleneck. These graphics are nowhere near the complexity of modern PC games like Call of Duty MW 2 or Civilization V and so you don't need a high-end card unless you're driving a 30" monitor or something.