In the face of discount, is the connection really necessary? I am about to go pick up a 500GB USB external hard drive at Target for $77.75. Will I be completely dissatisfied while I am backing up my data without Firewire?
If it is usb 2.0, it's not a huge difference. There have been threads saying usb 2.0 can be better than firewire or at least equal. An advantage of USB will be if you buy a newer MacBook without firewire you can still use thed riv.
Firewire has better sustained transfer rates. If you're recording and need to playback/record many audio tracks at the same time, you want Firewire over USB 2.0.
Backing up files however...it'll take a bit longer, but it's really not that bad.
IMO, if it's just for back-up, it should be fine. If you're going to be editing audio/video that's stored there, then you'd be better off with a FW 800 drive.