Internal HDD with Enclosure or "Pre-built" External?

I'm looking to back up my personal media collection (not huge right now, but growing rapidly) and am quite confused with how "external" hard drives are priced. I've been reading some topics, and it seems that people generally recommend placing a desired internal drive in an external enclosure. This seems to be fairly cost effective. For example, a WD 640GB w/ a Venus enclosure will run around $125 from the Egg. However, a 'prebuilt' 1TB can be as low as $150. How can this be?? Most 1TB internal drives start at $175... I'd like to get a quality product, so I'm willing ot pay a little bit extra for piece of mind, quality, and customer service (though if I lose stuff permanently, there is no way for the company to replace it).

Anyways, I am a complete ignoramus as far as this stuff goes, so any help would be greatly appreciated, and thanks for your time.

I've used both, and there isn't a lot of difference. If you want to get some swanky enclosure with pretty lights etc, that would be one reason to build your own. Another reason might be power consumption - I bought one of the Green Power disks that runs at 5400RPM and stuck it in an enclosure so I could leave it on all the time while using slightly less electricity. You might also have special cooling needs (or noise needs) that make you want to build your own. But for the most part, off-the-shelf parts work just fine.