Drupal 5.7 will do a nice job for you. Uber has not been updated for Drupal 6, but will at some point. Sometime after Uber version one is out. Right now Uber is at RC 4.

The other issue is that there are a number of key modules for Drupal that have not yet been updated to version 6 or sufficiently tested.

You will find Uber an excellent cart. Go with 5.7 and RC 4 and enjoy!

Jim