I think this thread needs clarity, just for sanity sake.
There are 3 types of external harddisks:
#1: The first type is usually sold by companies like Maxtor or LaCie. It's a single prepackaged harddisk that comes with nice, fancy enclosure (aka: outer metallic case) and a normal 3.5" harddisk installed inside. There's no installation or work necessary on your part. All you need to do is plug the unit into your computer. Since they use normal desktop HDs inside, they're usually at least 150 GB capacity now, probably larger.
The 2nd type is one you essentially assemble yourself. You need to buy a regular harddisk, buy an enclosure/casing of your choice, and put the HD into the enclosure yourself. It takes 1-2 minutes, and the only difference between #1 and #2 is that this method will save you money and likely give you a better quality HD for the same price. The capacity depends on the type of HD you buy, but since desktop HDs tend to start at around 150 GB to 200 GB, that's the smallest you're likely to get.
The 3rd type uses the type of HDs normally found in laptops and stuff. They're around 2.5" rather than 3.5", so they're smaller, and use very small enclosures. In many cases, they can be powered by plugging it into a powered USB port. Since they're small and don't necessarily need to be plugged in, they're more portable. You likely have one of these.
Do you, pyune, plan on taking this HD all over the place with you? I mean, will this external HD ever leave your desk? I have an external HD that I use to back up my photos and stuff, and it sits on my desk, and never moves.