dunno which model u got, but in the one i have, 42e or sumthn, it has a couple of usb modes... one of them is a usb streaming mode, so before u hook up ur usb to the cam, go to the play/edit mode, and from the options, find the usb modes thing, switch it to usb streaming, then connect ur cam. it'l work as a webcam, and ur cam's microphone would act as a pc microphone.... dunno abt the firewire method, thats a lot faster from wht i've heard... good for editing/ripping off vids frm ur cam's tape...
about digital cams, well, some of them, in rare cases have a webcam/streaming mode, some dont, so thats model dependant...