Laptops are really not gaming machines, I assume you have a mac laptop?
It's hard to say what graphics levels you'll be able to run at, untill you play the game and mess around and see what works best.
In my experience with weaker hardware medium/high textures low res character and world models /detail and high FX and volumetrics make games look pretty with out forcing your computer to process the extra polies.
I have an iMac, I think it's a 2008 one, so it's pretty old. Another thing that is bothering me is that the reccomended specs for diablo 3 indicate 12GB of free disk space. Do I really nedd that much?
i have a dual core amd and ati 5450 on my lap, and i can all max with 1024x768 resolution 30fps avg, so i guess ur machine can handle the game on low settings
the installed data is around 8GB, 12GB is required in additional for downloading future patches.