Dear Carlos,
I have a little experience in this area: I bought a MacBook Pro a few years ago (for a silent film gig!), and I originally partitioned the internal drive into three parts: system, projects and samples. This worked well enough, but later I wanted to run Windows in Boot Camp, and I had to un-partition the drive for this due to Boot Camp restrictions.
I then bought a Glyph portagig because it was 7200RPM and it needed no power supply. For some reason I was really reluctant to carry a power supply with the drive, even though they're not that big. I just didn't want the annoyance. While the Glyph drive is really nice, it's only 80GB and it was quite expensive, around $380 at the time, plus it is only Firewire 400.
If I were to re-do my setup today, I would probably do what Stephen Siegel suggests and use an eSATA card. I looked on macsales.com and found the Apiotech EC-0003D for $39. I don't know anything about this product and would personally do more research, but it looks very small and is cheap. It also fits into the slot on the MacBook Pro that I never use (thus freeing the two firewire slots). Then you could get a drive, like the Western Digital Quad interface My Book Studio for $196 for 750GB. I have two western digitals in my studio now, and the little power brick isn't that bad, it would easily fit in my backpack along with the cute little drive. With the money I save I could also buy a little portable drive for carrying large files, which is the other thing I use the Glyph for.
I just did a film project where the filmmakers bought me a little LaCie portable, bus powered drive. I mixed all the music for the film off that before I realized that it was only 5400RPM. Plus it only costs $99. My little Glyph is like the muscle car passed on the freeway by the Yaris. But hey, in the hard drive market today's bargain is tomorrow's regret. My little Glyph feels very unloved right now.