I own a PB G4 1.5GHz, all the bells and whistles. My graphics card is the ATI Mobility 9700 with 128 MB of VRAM. It has an S-Video out and DVI out.
Can I use either of these ports as an input with the correct software, or am I stuck with buying a USB Video input device?
I believe those ports are the equivalent of a one way street. You will most likely need to get a USB device.
Hell No!
The Canopus firewire capture devices are a reasonably good priced option ($200ish) or the USB2 ConvertX for a similar price.