I'm not sure regarding the limits as they would vary depending on engine capability/type of game (fps,mmoprg etc), though I think the building could be optimised lots by keeping the faces of the building flat and using bump or normal maps for the windows and corner columns, and the perimater on the top level could be a plane with an alpha map. It all depends on how close the camera is going to be to the details whether a bump/normal map would suffice and whether the building is a 'prop' or scene filler or is actually functional. And everything is triangulated in a game engine, no quads whatsoever. Hope some of this is useful
If you believe in telekinesis, raise my right hand.