The only credit cards which earn UA miles at a sensible rate are those issued by Chase. I suggest you begin by skimming this thread:
http://www.flyertalk.com/forum/credi...er-thread.html
...and reading the beginning, plus most recent posts, in this thread:
http://www.flyertalk.com/forum/miles...er-thread.html
Bear in mind that UA is a member of Star Alliance, and miles earned in any Star Alliance program can be redeemed for travel on flight operated by all Star Alliance carriers. This does NOT mean that miles can be moved between the programs, but it does mean that miles accumulated at (for example) US Airways and Air Canada can be redeemed through those programs for travel on United.