You would be better off with a MacPro, but quite honestly you should be fine with your iMac. CalDigit's FirewireVR was designed for video so it might be a good fit, and you can always use your LaCie for a time machine volume.
As far as the crashes, it's not easy to tell exactly what this issue might be. Step one trash your prefs for FCP. The second thing I would do is repair your permissions. Then I like to restart into single user mode and run an fsck -fy command. While these steps are sometimes considered voodoo it can't hurt anything. Then see if the problems are still there. If not great! If so you might want to consider reinstalling everything from the ground up, but that's a big step and not usually necessary.
There might also be some conflicts with software, but your iMac should be adequate. Keep us updated and good luck.
Jared PicuneIdea Spring Editing, Inc. Denver Final Cut Pro UGGeeky Mac | FCP Tips & Tricks
