You wont have much chance getting an upgrade unless Economy is very busy.
US Airways poilcy is to only offer upgrades to there Dividend Mile members, and you must usually have to have a Preferred or Chairman grade Dividend membership in order to qualify.
I flew with US from LGW
on way out and via CLT
on way home 2 weeks ago and the very helpful check-in staff at LGW
did enquire for me. I am a normal Dividend Miles member and said the airline would not allow me to upgrade, the claerk also mentioned that the "grade" of my ticket was not in my favour either as I had purchased a restricted fare. (Cheapest Ticket Available)
I wasn't dissappointed as I didn't expect to be upgraded in the first place, but you never know unless you ask!
If your flying on A330 economy is excellent, good legroom, p@ssport system, food not bad and comfy seats.
I would recommend sitting towards the rear - 32-35 rows are as good as near galley and get a very personal service from the cabin crew. Also the aircraft narrows towards the rear only 7 accross.