Many people will probably say "don't do it" but I say why not!
If you'e got the time and the inclination then there is no problem, and if you do you are probably of the style of the old school of computer users! You like to fiddle with things and don;t mind fixing it when things go wrong. Some people see a re-install as a big scary thing to be avoided. If you are inclined and know what you are doing then why not!
Anyway here is my backup routine which will certainly show you the sort of user I am.
1. Open up a teminal window
2. cd ..
3. tar -cvjf backup.tbz2 myuserdir
Then I copy it to another machine I use for backups, but you could just as easily burn it to a disk (though is might need splitting it you have a lot of files!).
Yikes! I would probably recommend that people who are not sure about how to backup their files to NOT start with Terminal commands.
If you have an external hard drive, just drag your Photo, Music, and Pictures folders into your external drive. Also, make sure to backup your iCal, Address Book, Safari Bookmarks, Mailboxes, etc.
If not, then, I would buy one. You're just throwing money away buying all these DVDs for one time use. Spend the $50 on a cheap hard drive, then you can backup everything and easily restore it.