Now, most people are (hopefully) out of New Orleans, so this water will probably just rinse away garbage and stuff, but this will definitely impede any rescue operations that are still ongoing. I can imagine that this will also cause stored foods and materials to be washed away, such as the Red Cross supplies or the established national guard command centers. The best advice anyone in New Orleans can follow right now is to stay as dry as possible and as high above sea level as possible; place all food and clean water in a safe spot. The best advice I can give anyone in Houston or Galveston: RUN!
I understand that Rita will hit later this evening (Friday) and will cause massive amounts of damage and carnage, despite the sea wall that will protect only ten miles of the Galveston coast. The sea wall only covers the eastern coast, so once it passes over and is on the west side, expect much much more damage to occur. But Texans are remarkable people, they will go on and thrive. This would be an excellent opportunity to compare two similar events and their aftermath. I fully expect to hear of little or no rioting and looting occurring within Texas, and if there is I expect to see that the looters are mostly Latino/Mexican. It isn't a matter of how the media portrays people, it's how it is in reality.