I'd definitely recommend a firewire drive.. USB may (supposedly, at least) be faster but i have always found it rather unreliable especially with audio work. Most audio developers recommend firewire drives to record to.
Id probably go with Lacie, or a G-Tech.
For lacie i'd go with one of these
http://www.macmall.com/p/LaCie-External-Hard-Drives/product~dpno~7335276~pdp.eccfhga
They look the absolute biz, as if you could throw my at a wall almost! lol..the only problem being that its 5,4K and only 8Mb cache. So its not gonna be as fast maybe..
http://www.datastores.co.uk/G_Drive_Mini/G_Tech_320Gb_G_Drive_Mini_Triple_Interface_7200rpm one of these drives might be better little bit more dosh though. but they are solid. and fast!!
Incidentally might wanna have a look at that website i got the link off for the G-Tech, they seem like they have a large selection.
http://www.datastores.co.uk- although no american equivelant unfortunately.
I'd not go with WD myself, outta the 5 drives that i have owned 3 went to pot, 1 had to get sent off the USB socket was dodgy-meaning that it would dismount on my mac. its replacement i use as a time machine for my laptop. WD's returns/cust. services are good though i will say that
Hope that helps
PTP
Aaaah didnt see the original post date.. lol