I have a friend that had to play it like that on a crappy laptop with a Core2Duo and Intel integrated graphics, at 800x600, and still only got like really unstable 20 fps. Went through the whole campaign and played MP like that. He still enjoyed it so more power to him I guess.
I played fallout 3 on a geoforce 750 and an intel p4 and got around 15 FPS out doors 25 indoors and beat it and still enjoyed it 15 is bothersome and would never do it again but 20 works even if its not consistent