PS - normally there would be a Firewire connector on the back of the system also - though it might be possible to have an internal connection which needs a cable/plate to bring it to the outside, this would be unusual.
Ah - I see there is 1394b (800Mbps) now offered - unless your computer says otherwise it will be 1394 (400Mbps).
You can get plug-in cards with 1394b (Firewire 800) - I'm pretty certain a camera with FW800 could be used into a FW400 computer - but just at the lower speed.
Whether changing your PC to FW800 would offer any major benefit, I don't know - clearly transfers would be twice as fast - but in practice whether this would matter, I can't say. FW400 is pretty fast anyway.