For a long time IEEE 1394 firewire was faster than usb (1.1) and a lot of camcorders and other high data rate devices used firewire. Sony calls it i.link (kinda funny that some one other that Mac uses "i" in a name)
Anyway this is from memory but
usb 1.1 had 14 Mb/s
Firewire 400 had 400 Mb/s
usb 2.0 had 480 Mb/s
Firewire 800 had 800 MB/s (hardly any devices yet that use this)
Again, those numbers may be off --- I use to work at an office supply store and this question came up often --- those numbers were from memory (don't wail on me if i'm off)
In answer to your question, if you don't plan on video editing (or have a new usb 2.0 camcorder) I wouldn't worry about it. If you want to be able to accept all forms of imput and have the $25 bucks, go for it. I know I use it but both my sony camcorders use this (but most newer ones have the option to go usb 2.0)
Follow up - for what its worth. I often edit dv material using my laptop with a USB2|1394 External hard drive attached. I can use the usb2 connection for data through put without problem. Cam firewired to laptop, laptop USB2ed to external hard drive for storage. As the hard drive makes my projects portable, I'm able to work on various desktops as well, using either input.
You should get 1394 one way or another. Onboard is nice because you can typically set front and back inputs on a decent case.
Thanks. I think I get what it is for. The idea to be able to add connectivity to this new rig appeals to me so I think I'll go ahead and get the ga-p35-ds3p. I really can't tell much of a difference between the two other than the firewire. Once again, thanks for the knowledge.