i don't know about Tiger, but when I insert a blank DVD-R into my MBP, it will ask if i want to show the blank disk in finder (in the side bar).
From there you can just drag the apps or programs/etc into the disk. I am not sure what you are wanting to save. I am sure you have some programs, but you would have the install discs, so that would be a waste of dvd/time to back up programs that you have the retail discs of.
I have backed up my stuff onto DVD-DLs. My itunes library (as of early July, 5900 songs) on DVD-DLs. I have an external HD as well, but those fail (eventually). I also have my pictures backed up. I will back up freeware programs, which might seem redundant, but sometimes (i reformatted a few months back) and forgot about some of the programs that i used. So for that reason, I will back up. Even programs that you don't get a disc with (ie; candybar, BBEdit, etc) I will back up, just because it is easier for me to do that, and sometimes i will be somewhere where i have no internet.
This will do until you get an external HD. They are getting cheaper and cheaper everyweek. I say a Seagate portable (fits in the pocket, almost) at OfficeMAx for $79. It was a 250GB. So I know you can get them cheaper online.