The biggest one you can stuff in there is 750 GB, since larger ones are too physically tall to fit. It doesn't matter too much which brand of drive you get, but keep the receipt for the warranty and make backups of your data.
It depends whether or not your data is important to you. You can't simply put the old drive into an enclosure and copy everything back over onto the new drive in the PS3. If you want all of your data on the new drive, then you need to use the Backup Utility to make a backup onto an external drive and restore onto the new drive once it's in the PS3.
Also, the data on the drives are linked to a particular PS3 and shows up as gibberish to all other PS3s and PCs. If your PS3 or hard drive dies and you don't have a backup, then your data is gone for good.