Skyrim can run on an intel 3000 HD, which I believe the 320m is about equal to. I'd say you can run Skyrim on low, and D3 should run, but I wouldn't do it on OS X.
in Mac OS X, the 320m is about equal to a HD3000... in Windows, its quite a bit faster. That machine can run both games... they really are different types of games though.