Actually, you dont have to make the box float, you just make it a detail brush.
Since the buildings you are describing sound like they are made up of many polygons, making them detail brushes also might help...
Additionally, where you place the buildings is important - and what is visible at any one point to the play is important with performance. Check around for tutorials breaking down vis blocking, hint brushes, etc... They will help you figure out how to get more frames out of your map
http://www.nibsworld.com/rtcw/tutorial_ ... art1.shtml
and read that website:>
-McBain
Day of Days Mod