I'm streaming The Singles Collection at this very moment, not sure why as I am not much of a Britney fan but I had a sudden urge to listen to 3 (one of my favourites of hers) and I couldn't for the life of me remember which album it was on. I thought maybe Blackout but it wasn't there (I recalled it came not too long after Piece of Me). I have to admit she really does have some good singles, except Slave 4 U which I can't stand - odd because Britney fans seem to think its wonderful.
As for the X Factor, they had Cheryl "lip synch" Cole as a judge so I see no reason why Britney should not do it. She is far more famous, not to mention a lot nicer. Pity its not the UK X-Factor as the last series was pretty dire with Kelly Rowlands and Tulisia.