I have a Macbook with 160GB internal HD here, and an external 200GB HD in USB enclosure.

The macbook isn't mine but i'm going to use for a couple of months, so i need to find an efficient way to backup it's contents, and then restore them back when i return the macbook to it's owner. This is because i want to use it with 10.6 and currently it runs with 10.5.7.

Would it be a good idea to use Time Machine for backup purposes, or maybe clone it via Disk Utility? Preferably i'd like to make a bootable clone, but i have to use the safest way possible because i can't lose any data.

Try CC (carbon copy cloner). http://www.macupdate.com/info.php/id/7032

Does it makes a bootable clone?

On a Mac, YES!!

SticMAC
ps,; You can also do this for your Hackintosh(if you want) by just loading Chameleon, the few patches, then booting from it!

Thanks!