From Tiger to Leopard in 10 steps
1- Get an external hard drive, in my case a 120Gb 2.5" Western Digital laptop-like IDE disk inside a USB 2.0 external case.
2- Get rid of all the unnecesary stuff in my actual macacosx installation (a lot of rm -rf of directories like screencasts, mp3, anime, etc and a lot of scp -r of some other directories like codigo23, e-shell.org or backgrounds).
4- Use Superduper to create an exact, bootable copy of my actual Tiger installation (so, in case of trouble I could boot from the external hard drive, or use Superduper again to get the system fully restored). Important: You really should get a firewire external disk instead of an USB one (as superduper recommends) if you are in a hurry, in my case, even using USB 2.0 it tooks sooooo long, you have been warned.
5- Use the bootdisk option from the system preferences in my actual Tiger installation to boot from the cloned system in the external USB hard drive. Once it boots up, check everything is ok and use the bootdisk option to boot again the Tiger installed in the macbook hard drive.
6- Put the Leopard installation DVD inside the Superdrive and reboot the laptop (do not forget to press the C letter in the keyboard, so it boots from the DVD! (thnx Clayton!).
7- Follow the on-screen instructions (in my case disabling installation of unneeded languages like Japanese or Italian)
8- Wait 30-40 minutes
10- Finally install all the needed software, like Carbon Emacs, Macports, Firefox, Opera, Burn and a lot of widgets. Of course, put back in there all the data from the backups.
I've to say that the process was totally smooth and unpainful and everything seems to be working just fine. Hope that the fact Leopard is a 64-bit operating system will make the difference...