Games larger than 4g will have to be split in order to be copied to a FAT32 drive.
Once you choose to backup to the external using your manager, some will do the split automatically ,like Gaia or open manager.
However most games CAN NOT be played from external when they are split. So you will have to transfer them back to your internal (joined automatically again)with your manager.
Which means that depending how many games you play at the same time, you might need to upgrade your internal anyway unless you are willing to transfer back and forth all the time.
I chose to get an external for now as I back up a lot on my PC and find it easy to plug the HHD to it and then transfer to the PS3 instead of using wireless transfer... You can use PS3SPLITTER for the PC to break up large files and put them in your HHD, and it can also be downloaded from this site.Once you have games backed up to your drives, just use a manager to load them and play.