For top end games, so far I've just played Starcraft 2; and well, it did work. I ran it at native (crazy, I know) resolution with all the details turned down to lowest. Started up a 4v4 coop game with the AI and off I went. At best, FPS was in the 30's. Towards the end with lots of visible action, it was dipping down to 10-15 FPS. But I have to say, it was technically playable. Although next time I'll turn down the resolution for a bit more frames. I'll be trying out Civ 5 later.
Oh, and yes, it will get hot. External temps were around 108 degrees at the top/right and the fan was spinning away.