Usually games will have a text file that saves these graphics settings. In this file, there are many advanced settings that can be changed to further reduce the graphics that the in game menu doesn't offer.
I would recommend having a friend who owns D3 or is in beta play it on your PC and look/research for this file and see what graphic settings can be lowered/turned off so you can see how it will run on your machine.
yeah, that or pirate it for a test run. Though it might have a demo available (starcraft II does).