How much does N.O. have to worry about the upper Mississippi getting any extra volume from Rita when she moves up? Any?
Depends on how long Rita lingers.
Here is national link for river gauges.
http://www.weather.gov/ahps/
From there you can navigate.
AFAIK none of the River levees broke in Katrina, and they are REAL levees, not the POS concrete slab that gave way on the canal. They should be okay. River is lower this time of year also than in Spring.