PCI is for the PCI slots in the back of a desktop computer. Cardbus is for the little credit card sized "PC Card" slot in a laptop.

USB is 12mbps, with massive protocol overhead (which is why synching your Empeg is faster w/ 10mbps Ethernet than 12mbps USB); Firewire 400; USB 2 is 480, but will be slower than FW in real use, and Firewire actually exists.

You can get a Firewire enclosure for a 2.5" or 3.5" drive for about a hundred bucks, then put whatever drive you want in it.. if you're not going to be carrying it around, go 3.5" since they're much cheaper. Or for somewhat more than an enclosure + drive you can get one that's already built if you don't want to deal with putting the drive in the case.