I'm a 1500 rated player so I don't worry about rating too much.  I was wondering if the rating adjustment is set at the beginning of a game or does it change as my opponents rating changes?  For example, I start a game against XXXXX who has a 1490 rating.  When I win the game XXXX's rating has increased to 1650.  Does my rating reflect beating a 1490 player or a 1650? 

It changes based on the ratings at the end of the game.