Yep, basically, for normal capture a firewire port is all you need. My experience tells me that the only reason for an capture card is to capture analogue (from a VCR or 'old fashioned'

camcorder).
As for the firewire port though. One thing I'd just like to add, and this is not personal experience, just forum struff I've read out and about. I have heard of people complaining that firewire capture doesn't work and that the problem has been solved simply by not using the firewire port(s) on the Mobo but by investing a small amnout in a seperate PCI firewire card. I've seen this solution a few times crop up so do be aware of it.