Users/Guides/Backing Up OS X
psync is one option, though I had some trouble with the perl prerequisites.
Another option is to use Carbon Copy Cloner which can make a copy of your OS X drive onto a Firewire device such as an iPod. You can then boot from this device, partition your Intel Mac's hard drive and use the program to copy OS X back to the computer after partitioning. There is no data loss and no need to re-install OS X.
For more information about how to back up with Carbon Copy Cloner, visit the How_To_Backup_your_Mac page
Superduper! is another alternative to do smart backups and full bootable backups of your OSX partition. Superduper! is also a universal binary which makes it a much faster backup option to Carbon Copy Cloner.
If you need to decrease the size of your OS X drive before installing, you could use the open source application Monolingual to remove (non-US) languages, which you don't need (this app frequently removes more than 1.5, sometimes 2 GB of unneeded data) , and you could temporarily put your Documents and Applications onto DVDs (as long as this won't interfere with OS X when you boot from the Firewire device).
BE VERY CAREFUL WHEN USING Monolingual. Removing PowerPC part of the system is not recommended, since every PowerPC application and Rosetta requires the PPC code to run properly. Do not remove the English language too, since this is required by most applications. These problems are fatal and you might need to do a complete reinstall is something goes wrong...
Still, for specific applications you could use TrimTheFat, which is able to produce a backup of your application, for safety reasons.
Another (shareware) application is Tinkertool System to remove unneeded languages.
If you wish to backup your system with terminal commands, you can visit the How_To_Backup_your_Mac page too. . . . .