Firewire or USB interface?

You can use either

There are plenty of USB interfaces out there, plus a handful of Firewire interfaces.

Edirol and Tascam make USB stuff, fairly inexpensive. Edirol just came out with a Firewire box too, but I haven't seen it in stores yet.

Digidesign makes the MBox (USB) which is around $450, but includes ProTools LE and some quaility pre-amps to boot. They also make the Digi 002, which is Firewire-based, and costs $1200-2200. The MBox is portable, the Digi 002 isn't.

M-Audio makes both USB and Firewire devices, Firewire being more expensive and more capable. USB runs from $150-300, Firewire $300-500.

There are other manufactures like Mark of the Unicorn who specialize in Firewire stuff. It's good, but most of it is rack-mount, so it isn't terribly portable.

One thing. USB interfaces can have latency issues while Firewire generally won't, though it appears Core Audio and better drivers from the manufacturers has made latency less of an issue than before.