You won't be able to have 60fps even on main character with this hardware, sorry.

I have i7 4970k and 2x970 in SLI. In Diablo 3 video settings everything is on low. Active character locked at 60fps, background characters locked at 30fps. Still, in intense fights, FPS drops to low 50s on active character. Oh, that's in 2560x1440 resolution.

Most likely your video card is a bottleneck. Try making your windows smaller (change size in ISBoxer, open D3 video settings, Save) and see if it improves performance.

On other hand, if you can get stable 30fps on active character - that's very playable.