Here's what's flooded right now, 230 pm EDT, per NOAA: at plus five feet ASL:
At plus 13 feet ASL, about 80 percent of Galveston goes underwater:
http://i4.photobucket.com/albums/y138/jeffers_mz/Ikeplus13ASL.jpg
At plus 30 feet ASL, almost of all of Galveston and sigificant parts of the coastal metroplex are submerged:
http://i4.photobucket.com/albums/y138/jeffers_mz/Ikeplus30ASL.jpg
Here's a topographic map with five foot elevation increments:
http://i4.photobucket.com/albums/y138/jeffers_mz/Ike1.jpg
Wave height plus tide over mean sea level will add to the surge levels displayed.