iMac Hard drive solutions.

Hey guys, i'm looking to 'upgrade' my cousins iMac 21.5" version with a Large storage HDD. (not sure how big yet)
Today i've given it some thought and am getting some second opinions on which road to take.
1: Firewire 800 Enclosure with internal HDD.
2: Upgrade the existing HDD to a larger size.
3: (If possible) install a secondary HDD
i: Is there a second slot available
ii: Is there a walkthrough somewhere on how to do it?

Any help is greatly appreciated.

Ba!nesy

http://www.ifixit.com/Guide/Repair/iMac-Intel-21-5-Inch-Hard-Drive-Replacement/1766/1

I would suggest a large firewire drive. It functions well as a time machine backup, and keeps you from having to mess with removing the glass front.

Firewire is AWESOME.