The FAQ section has a wealth of information on most any topic of interest. But, here are the key links related to your question:
US Airways Flights for United fliers
Upgrading on US Air
http://www.flyertalk.com/forum/showthread.php?t=112410
Upgrading on US Air - other scenarios
http://www.flyertalk.com/forum/showthread.php?t=342837
Earning 500-milers/CR1's on US Air
http://www.flyertalk.com/forum/showthread.php?t=328388