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I can't give specifics as I tried it when I first got my Mini at launch. From what I recall I had to put the settings on medium for it to run well. On medium it gave a decent enough frame rate for it to be playable.
 
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.
 
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.

That's very similar to what I have in terms of specs and settings in the game.
 
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