According to this NY Times article, apparently not much:

Six inches.
After two years and more than a billion dollars spent by the Army Corps of Engineers to rebuild New Orleansís hurricane protection system, that is how much the water level is likely to be reduced if a big 1-in-100 flood hits Leah Pratcherís Gentilly neighborhood.
Looking over the maps that showed other possible water levels around the city, Ms. Pratcher grew increasingly furious. Her house got four feet of water after Hurricane Katrina, and still stands to get almost as much from a 1-in-100 flood. [ . . .]
New Orleans was swamped by Hurricane Katrina; now it is awash in data, studied obsessively in homes all over town. And the simple message conveyed by that data is that while parts of the city are substantially safer, others have changed little. New Orleans remains a very risky place to live.
The entire flood system still provides much less protection than New Orleans needs, and the pre-Katrina patchwork of levees, floodwalls and gates that a Corps of Engineers investigation called ìa system in name onlyî is still just that.
The corps has strengthened miles of floodwalls, but not always in places where people live. It has built up breached walls on the east side of one major canal, but left the west side, which stood up to Hurricane Katrina, lower and thus more vulnerable. It has not closed the canals that have often been described as funnels for floodwaters into the city. [. . .]
As a result, the city still lacks a system that can stand up to that 1-in-100 storm, let alone one like Hurricane Katrina, which the corps calls a 1-in-396 storm. The work that could build the more robust system ó originally estimated at $7 billion, and now at least twice that ó will not be completed until 2011 at the earliest, and experts agree that even that level of protection will be less than the city needs.
The corps is working on a two-year, $20 million study to find ways of providing even more protection, but it will not even be released until December.

Read the entire article. As noted in many posts over the past two years in the hurricane category of this blog, the performance of the various federal, state and local governmental entities in rebuilding New Orleans has been generally abysmal, at best.