If you cruise over to Pricewatch and search on "usb 2.0 disk case" you'll be presented with a host of options. The cases do indeed take a standard IDE drive, I have two of them sitting to the left of my feet with 120gig drives in them for backup. Some newer systems do indeed have USB boot capability, and they'll boot from these drives. I see one for $44.
Wait, here's an ad for the one I have USB 2.0 Case for $49. 