I have a 640gig hd in my mac, and about 60gigs of space used. Would an external USB 2.0 120gig hard drive be enough for me for use with Time Machine if the only part of the 640gig hard drive that I want to back up has to do with my work files which take up roughly 10-20gigs of space? Should I go with a bigger drive instead? Is FireWire really needed? How about a dedicated power plug (the drive in question is USB powered).