I have to play devil's advocate on the firewire suggestion.
Unless you plan to move mass amounts of data often, not just initially and smaller stuff you accumulate here and there, I fail to see why firewire is so necessary. I have 2 USB MY Book drives and accessing info and files on it is instantaneous. I even watch full DVD rips on my MBP that I keep stored on my 500GB drive, and don't have any problems. If I had sprung for the extra speed of firewire, it would be totally wasted capability.
Unless someone can explain what the real advantages to the OP'er would be to have firewire in his situation, I say save some money and get a USB connected drive. You can get a 500 GB My Book for Newegg for $124 here.
But if you're bent on getting firewire. There's a cheaper option with more capacity here.
Oh, and if you decide to head back to a portable option, you can get a 160GB version of the same drive you were looking for, for $5 less here.
No disrespect, but despite what one person says and WD drives, many, many people use them and are quite happy with their performance, myself included.
I believe you need at least a Firewire 400 to be able to boot from an external hard drive. This is an important feature for me but NOT everyone.
Yes, USB 2.0 is faster than Firewire 400 (480Mbps Vs. 400Mps or 60MBps Vs. 50MBps).
I was going to order OWC Mercury Elite-AL Quad but decided to get http://www.4videoequipment.com/video/store2/Scripts/prodList.asp
G-Tech Q Drive instead.
Both drives are very good too! The OWC Mercury is available bare bones so you can add your own drive which would keep it in your buget too. I paid more just because I'm vain and like the way the G-Tech looks