Diablo 3 is well optimized. It's less demanding than Starcraft 2 and here's a video of the same APU running SC2:http://www.youtube.com/watch?v=m8RUTrlpEHg
So Diablo 3 would actually run nicely on that laptop (though I'm not an Acer fan). I saw a 17" with this APU at Bestbuy.com for only $450 (forget the brand).
You probably want to tweak Diablo 3 to get roughly 35 FPS with VSYNC off then enable it to force 30FPS. Diablo 3 has a maximum FPS option but I'm not sure how well that works with VSYNC.
If you can't get VSYNC to work at 30FPS properly, you can use the RadeonPro tool and use the "Tweak->Double VSYNC" option to force 30FPS. I did that for The Sims 3 and it forced 30FPS with VSYNC nicely.