I've heard the Viva Piñata games are really good and at least two of your options (Perfect Dark Zero and the new Destroy All Humans! game) are terrible.
What retail games do you currently have? You could probably pick up a few great older games you don't have for the same price as one new game (including the first Viva Piñata, so this option would work nicely). Also, have you considered any XBLA games/what ones do you have? For the cost of a new retail game, you could buy twelve 400-point games, or nine 400-point games and Braid, or both Geometry Wars games, Braid, and six other 400-point games, or...