Backing Up

So before I upgrade to Leopard, I'd like to do a backup since a number of people have lost everything. My question is - how do I do it? I don't want to have to go out and buy an external HD just so I can upgrade to Leopard. Not do I really want to spend money on a program to backup my data.

I'm going to use the Archive + Install option to upgrade but just to be sure, what is the best way to backup? And will the backup fit on a DVD?

I have Deja Vu but have never used it. Will this do the job for me?

Bumping because it seems like a reasonable question that quite a number of people would know the answer to... but nobody's replying =\

I've read nothing here about anyone "losing everything" per-se.

I have no advice about backing up since I didn't back up any of my own systems before upgrading via A&I.