I think that United is trying to move away from the 747; for that reason, I wouldn't expect to see any additional 744s for United, unless they got a very good deal on them.
As for any 772s, IIRC United was the launch customer for the 777 so you're talking about the oldest 777s out there. They are underpowered by 777 standards and I don't see United wanting to add any 772s.
I think that they're still debating on which widebody that they will get for the future. I wouldn't be surprised to see United move to an Airbus widebody.