try to disable all the graphics settings, the world details etc like in this screenshot
also reduce the resolution to 800x600 or 640x480
and you can put the gore level to "No Gore"
http://cluex.org/img/?di=AHTD
I tried it and now i always play like that. I used to have 4 gb ram and a nvidia geforce 8800 gts and play with 200-250 fps but one pin of ram (1gb) died and even my graphic card. it happened the last winter. I always been used to 1920x1080 and highest graphic details but it became unplayable ingame (around 10 fps) without the graphic card.
Now with everythings disabled and turned to lowest/low and resolution on 640x480 I've got around 100 fps.
by the way, if you don't know how to show your current fps: press tab and type: stat fps



