Question about the ps3

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I am going to be getting my husband and my son the ps3. I dont really understand the difference between the 20gb and the 60gb. Is 20gb and 60gb memory? If so what is the memory for? The only thing I can think of is its for saving your games. Is the 60gb worth the extra money? If there is anything you can add please do. I just want to understand it a little better before I go out and buy one. Thank You, Denyel