Any ATA laptop hard drive will work. I really don't have recommendation on brand. But whatever brand you go with, get something that is faster than 4200rpm. That way you will get a slight performance increase.
I replaced my wife's g3 ibooks with a 60 gig that was salvaged from a older windows laptop. When you go to re-install os-x you will have to run the Disk Utility to initialize the new drive.
The only problem is getting to it. There is a crap load of screws of various sizes that you will need to keep track of. Check out
Powerbookmedic.com for a guide on how to do this. It is not for the faint of heart.