Tall isn't the answer, dense is. The proper question is "when you give up 1km² of farmland, how many beds and offices do you get in return?" (or a little more sophisticated). Tall doesn't actually correlate very well with dense, because it tends to be a small part of the ground.

IIRC Bldgblog wrote that the US has about four ground-level parking spots per car. (Can't find the posting again now, but http://www.bldgblog.com/2007/10/greater-los-angeles/ and everything it links to is a fine place to start.) If you build some highrises, plus big flat malls and office parks, plus four parking spots for each car owner that's expected to move in, plus enough streets, then the highrises don't matter. You could build them half or twice as high and it wouldn't make a real difference to the density.