Couple of things.
First, I've never run DAO on a 330M or 320M so I can't even guess at performance. Ok well yes I can guess, I guess it would probably run well enough to enjoy playing.
More substantially though - I suggest you seriously consider buying and playing the Windows version under bootcamp.
I feel bad recommending a Windows over an OSX version because we all want OSX native ports; but the truth is the Windows version runs better (partly just because Windows video card drivers are updated much more regularly than the OSX ones).
There is a second possibly even more important consideration - the OSX version has only limited support. Some of the updates and DLC content don't work on/exist for the OSX version, or can only be made to work with unsupported messing about. If you are happy just to play the vanilla game, or you have something against Bootcamp, the OSX version works and all. But if you want access to all the goodies now and into the future, the Windows version is the safer bet. Sad but true.
Great game btw especially if you enjoy the RP. Have fun!