That's very similar to what I have in terms of specs and settings in the game.I am pulling a regular 27 to 30FPS on my mini with the 6630.
My machine has 8GB of RAM and a 750GB 7200RPM Harddrive. My settings are on medium, 2xAA, Shadows off and terrain detail at good. When I am running a 5man or a good deal of people are on the screen with spell effects cranking, I fall into the high teens. I should note I have my rez cranked up to 2560-by-1440 (what can I say I like my cinema display). The game is playable and you can do better than I am by sacrificing things here or there.