I've been very happy with my 250GB external firewire purchased from
www.macsales.com. For important, irreplaceable things like pictures of your kids, I'd also suggest online storage--it's pretty dang cheap, and that way, even if your entire house were (a) burned to the ground, or (b) everything stolen, you can still retrieve your precious photos.
My (overly paranoid) solution: photos on internal hard drives on both machines, and on two external hard drives; also backed up onto DVD-Rs which I store at my office at work.