There's an issue with some cards (especially AMD iirc) in that the game only uses a small percentage of the GPU resulting in frequent and nasty fps drops.
The solution is to simply enable the FPS unlocker in DSfix and set it to 30/60. You can also try forcing anti-aliasing through your GPU control panel.
Alternatively, there's also this http://forums.steampowered.com/forums/showthread.php?t=2902805 which I haven't tried.
Yeah, I have an AMD APU. Wasn't expecting it to run great but I was hoping to at least get 30fps even at a lower resolution. I do have the fps unlocked already but that hasn't helped much.
Well on my laptop I use it to decrease the horizontal resolution so it improves performance with my normal PC downsampling from a higher resolution. I have it running at about 800x720 on my laptop since it crashes if you lower the vertical resolution further than 720 for some reason.
I lowered my resolution but it didn't seem to affect the framerate.