Well, depending on your needs, you can get up to 1TB if you have a unibody model, or you can get a slightly faster 750GB drive. I'd personally recommend the 500 or 750GB Western Digital scorpio black.
Just pay attention to the spindle speed and cache buffer, and of course size. If you want MBP portability take into consideration the power usage as well as it pertains to battery drain.
Don't forget the warranty. It doesn't matter if you change the harddrive out two years down the road and use it as an external and it craps out on you, you get another drive from the manufacturer (at least that's the way it was) as long as it is within the warranty period. Too many people just buy a new one and trash their old one instead of using the warranty.