As the name implies, the Fireface is a Firewire-based interface; and in addition to being RME's first stand-alone external Firewire device, it's also the first Firewire 800 (IEEE 1394b) interface on the market. This doesn't mean, however, that you can't use the Fireface with a Firefire 400 (IEEE 1394a) interface: the Fireface is completely compatible with 1394a interfaces, and, in fact, the unit itself is supplied with a Firewire 400 cable. For this reason, since Firewire 800 is backwardly compatible with Firewire 400 anyway, you might be wondering why the Fireface being a 1394b-based interface is so significant. Well, in keeping with many of RME's other products, it's possible to connect multiple Firefaces to the same Firewire buss — up to three, in fact — but only Firewire 800 offers the necessary bandwidth to do this. So, in a nutshell, if you'll only ever want to use one Fireface, Firewire 400 is fine; if you want to use multiple Firefaces, you'll need to use Firewire 800.
Mel