Would a motherboard (one bought today) that has Firewire port be a 1394a or 1394b.
You must check the specifications on the manufacturer's site. Google the motherboard's name and you will get there fast.
If I choose to buy a PCI Firewire add-on card should I get the PCIx16 or does it matter if I just use PCIx1.
You mean PCI-e. Anyway I said already. if the PCI-e slot on the motherboard is PCI-e 2.0 you can get a x1 card and it will be fast enough for a
single firewire 800 (1394b) port (even if it has more than one port). Again, check from the specifications on the manufacturer site.
If you just need to run firewire 400 (1394a), a PCI card will be enough
Anyway, don't confuse PCI with PCI-e, they are different in both appearence and speed.
THis pic shows a PCI slot down below, and 4 sizes of PCI-e slots.
This is a PCI card (USB ports).
This is a PCI-e x1 card (ethernet). Look at the size and shape of the part with golden lines.
What is the difference between a ePCI and a Full PCI port that fireberd talked about?
Full PCI port is a port that is controlled by a PCI controller in the chipset or somewhere on the board, but most modern motherboards simply convert a PCI-e slot into PCI with conversion chips and solder a PCI slot in its place, and that may cause issues with some cards.