With a 720p monitor, you will see no performance decrease.
For the sake of the argument, lets say something does demand more than 1 gb at 720 resolution, What would happen then? Since you have no shortage of RAM, the card would begin to utilise shared video memory; It begins add on to your 1 gig of video memory, by using up some of your RAM. This will not be as fast as the 2gb card, but the performance decrease is minimal. Again, nothing will use more than 1gb at 720, I'm just trying to dAmpen your fears