I have just replaced my old Mac Pro with an 8-Core (2x3.2 Ghz Quad Core Intel Xeon with an ATI Radeon HD 4870). I had recently bought a new Hard Drive for my old machine which I had a clean 10.6 install on and then added FCS 3. FCP is working fine but Motion will not open at all. I get to select what format I want to work in then it crashes before opening any Motion windows.
I have trashes preferences, re-installed and done software updates to no avail.
As I read this you moved the hard drive from your old Mac to the new one - correct?
In theory this should work just fine with MacOS X (and it sometimes does), but in reality you are setting yourself up for a world of hurt.
Do you have an empty drive in your system? If so, try doing a clean install of OS X and FCS on that and boot from it. If that works, I'll bet you that the problem stems from moving the drive over.