you could try firewire, or get a usb external harddrive kit, but for both of them youd need an ide harddrive to boot off of, and with firewire youll need to buy a card to add ports if you dont have them. Ive never heard of a ide->usb adapter, and im not really sure how possible they are.
and this link: http://www.deltrontech.com/Enclosure/ESlim/ESlim.htm
this product looks super cool because it has a battery built in, so you don't have to get external power to it. Plus when you aren't using it, it charges the battery from the power off the usb or firewire cable. If you don't want the external enclosure I'm sure you can do some modding