Some advice:
I recently purchased a mybook pro 500 gb with the firewire800/usb interface.
It holds a lot.
Using USB it is slow. Using FW400, it is reasonably fast. Using FW800 is pretty nice.
But it is loud from fan noise and vibrations when it is not sleeping. I'm not terribly impressed with the read/write speeds either.
It's fine I suppose.
I will say that I'm more than impressed with something else I just bought:
a seagate 7200.10 ATA hard drive (300 gb) and a 25 dollar firewire/usb external enclosure with a fan and external power supply.
The Seagate drive is much faster than the one in the WD mybook pro. It is also quieter and seems to be more responsive in initial activity...it doesn't hang when it's been unused for awhile and gets some attention suddenly, which the mybook does connected to the same machine.
It is really very quiet and I can only just barely hear the case fan. I got my enclosure from newegg..."Anywhere" was the manufacturer name. I just wanted a cheap enclosure for a drive I had been using as an internal drive (the seagate 7200.10) in my PC, and this fit the bill. Turns out it's better than the mybook pro as long as you don't need FW800, and from what I can tell, it's more responsive even compared to FW800 when you look at random access, because the drive itself is faster. I don't think the mybook pro's drive is as fast as the FW400 interface in most situations, so having FW800 support is nice but not necessarily important.
newegg has the seagate 7200.10 (ST3500630A) for 119.99 right now, which is the ATA version of their new 500 gb drive.
The also have the Anyware ME-720U2FSV, which is USB 2.0 and FW400 with an ATA interface and a fan. It isn't the most beautiful thing in the world, but it isn't terribly ugly. It's easy to set up and effective.
I'd go with that if I were you. I've been very happy with it so far...