To answer your question... use GUID partition table.
You can still clone your existing hard drive even if you don't have the OS X restore disks. Get a copy of
Carbon Copy Cloner and then connect your external disk, run Disk Utility and select your external drive, and erase it, choosing MacOS Extended (Journaled) format. Then, run CCC and select your source disk (your Mac HD) and your target disk (the external) and do a "backup everything" clone. The first time you do this, click "delete items that don't exist on the source" and it will do a complete copy of your hard drive which should be bootable. When this is finished (could take a little while depending on how many gigs have to be copied) reboot your Mac from the external disk to make sure it works. Once you're sure it's a good clone, you can swap the drives out if you're upgrading your internal drive, and should be good to go. If you're only doing it as a backup, then you can just keep doing incremental backups to the clone. It's fairly self-explanatory once you're doing it.
I'd recommend using a bare drive in an external enclosure rather than an external drive if you're going to install the cloned drive in your MBP, because if you take apart an existing external drive, you'll void the warranty on the hard drive, although it would work.
Good luck.
PS: What happened to your restore disks? They can be pretty handy, so maybe you should get new ones from Apple.