What was your average fps? I'm getting a similar GPU to yours.
Sorry for the late answer.
On my PC the average framerate was...I don't know, maybe 15 fps max. I used the same graphics settings as I did on my laptop (see page 1 for that) except that I used a resolution of 1680x1050 and an internal rendering resolution of 900p.
Not only did the framerate stutter like Porky Pig tripping on acid, it also had various graphical glitches which probably resulted from strange things during loading. For example during the first in-game cutscene where Nick opens a trunk of a car, there was no car.
Yet when I got to play the first section where you gather food I couldn't pick up the bagel because a car mysteriously spawned on top of it as well as three zombies on that car whcih didn't do nothing except look at Nick.
I played it on my PC until I got to the Diner, then turned it off because I couldn't play the game at this framerate anymore.
20 fps is one thing for me but frequent dips to 10 or lower fps is absolutely unacceptable.
I also played the game on my laptop again after that (see page 1 in this thread for specs) and after turning off Subsurface Scattering it ran smooth as fuck. 30 fps everywhere (except cutscenes, these still bug out on the framerate). Really nice and smooth.