with 80 Stream Processors 1GB 64-bit DDR3 at what resolution did you play ? 800x600 or 640x480 @ lowish fps with low settings i think you better buy a new pc or play it on a console.
i am playin on 1366x768..revelations worked quite smoothly but with brotherhood i had some issues with sound not matchin the lips of the character ..i dont exactly know the sttings but i am preety sure dat they wer medium..
Give it a try. You may have to reduce some eye candy. Only you can decide!