You're on the right track now, the processor makes the biggest difference in FPS. Unless your running shaders graphics hardware makes next to no difference (maybe 5-10 fps at most) and anything over 512meg ram makes almost no difference.
Even a smelly old P4 should manage 25-40 fps. Does it improve if you leave the game running for a few minutes? (TB runs slow when it starts)
I personally think there's something chewing your processor in the background.