Both Sony and Canon sell miniDV cameras for around $250. They are standard def cameras, possibly with a 16:9 wide-angle option. They should be just plug and play via the firewire ports. You can buy a lot of tape for the remaining $150 and they are good backup medium. Firewire cameras can also be used at a second iSight web camera.
If you can find a camera that will work with your Mac that does at least 720p, I think that will be more enjoyable because of the higher density picture. These new HD cameras are amazing, even the very low end ones like the KODAK Zi8. The low end cameras have no, or terrible, steady shot capability, so a tripod is even more advisable.
I can't think of a reason you need a firewire camera. You machine can read from the many USB cameras too.