What is the criteria for getting gold stars in Rock Band 2?

Two points have to be met in order for you to obtain gold stars:

• You must be playing on Expert difficulty (all instruments if there are multiple players; if guitar, bass and vocals are on Expert, but drums are on Hard, you won't get gold stars no matter how well you perform).
• You must achieve a certain number of points (generally around 1.4x the 5-star score in RB2); this can require taking an optimal overdrive path through the song, but doesn't necessarily require getting 100%. For drums, you can find a list of optimal activations here, and tips on improving your score here. For guitar/bass, you can find a list of paths and the scores different score-improving techniques will optimally net you here. For vocals, here are the paths for RB1, RB2, and DLC. This site shows you the cutoffs for each star ranking on the instruments and difficulties of your choosing for all games/DLC; put the instruments on expert to see the gold star cutoff.