I guess that if You have 2 monitors (now I see why people have more than 1) and enough RAM You can play another game on the second monitor like FPS, racing, maybe even something online just waiting for the AI's in Civ make their turns !!!!
btw. Is it possible to open 8 different instances of CIV for each core ? or play CIV 1,2,3 and 4 simultaneously on 4 monitors !
Good question! I know I've accidentally opened other games while Civ3 is in the background before, to no ill effect. I haven't tried having, say, Civ3 full-screen on one monitor and Civ4 full screen on another so you could play both without alt-tabbing between them, though.