Wait if you already have the hard drive and the firewire enclosure then you don't (necessarily) need to spend moar cash.
If you don't then , yeah, OWC enclosures are good; uses Oxford Chipsets and all that good stuff (crap firewire chipsets will cause problems).
If you already have the drive and the cable then programs such as Time Machine, CCC, Superduper ($), Retrospect ($$$$$$$), Data Backup 3 ($$$), and others will start to backup your machine.
You have oodles of choices in how to backup your data if the amount of data you need to backup is less than your storage capacity. You can do full, incremental or a hybrid approach depending on your needs.