Just to add to Stephens post; the PS3 is designed so that the internal hard drive can be easily replaced and when a new one is installed it will automatically configure it ready for use. As Stephen said, all you need to do is backup your data (Game saves etc) and restore them onto your bigger drive.
Here is a good guide on what needs to be done -
http://uk.gamespot.com/features/6176090/index.html
Here is a video -
http://www.youtube.com/watch?v=AuyQPW3bDS0