Believe me Joe, I think Marty has chosen the correct option already in going for an external hard drive.
The talk of DAT's is rediculous nowadays, even for corporates, given the low cost and ease of use compared to hard drives and reliability of SCSI.
Using a DVD to back up is all well and good, but its tedious, DVD's can be unreliable at the best of times and they are easily scratched and damaged. Also, you can end up with loads of DVD's (even DL) when it could all be catered for by one HDD.
The kits to convert internal HDD's to external ones are good, but given the cost of external HDD's nowadays, there isnt that much need for them. Also, its simpler and safer to transport/carry an external drive then an internal drive.
Only before I was looking at internal drives in Bidston Tesco, and they have a decent range in stock for a Tesco lol. Noticed a 250GB Freecom for £99, which to be fair, considering Tesco arnt really a PC retailer or anything, I though was very competitive even with online retailers.
I am even thinking of getting one myself at that price!
And of course it has the added beauty of being able to be returned if its broken. A lot simpler then any type of RMA service.