Downloaded ps3 games and multiplayers?

I am looking at purchasing and downloading ps3 games from the psn. Am I better off just buying the game at the store for a lot more money or is it the same if I download it? And if I download, how many games will the 80gb hold? Also, we have 4 people in our family and have 4 controllers. What games can we all play together at the same time? And can all of us play the games that we would download? We did download the free demo versions of a few, but they are all one player and don't allow you to do a lot. We have tried to look for multiplayer games for 4 players and there aren't that many to be found. Is there anyway that all 4 of us could play online at once? Any information on this would be helpful Thank you.