I just tested this on my quad so here is your answer...
right click or option click your FCP icon and "show Package contents" Nav to the info.plist there, and open it in text.
It might be good to make a backup of this file just in case.
At the top of the plist entries, you will see something like this...note the <string>PCI</string> I already hacked this to run on NON agp macs... ;-)
Anyway, all you need to do is delete the entire block of text listed below, save it and run FCP 4.5.
These keys are not needed to run FCP at all, they just are ther to give you headaches...hehe
<key>AELMinimumRAMSize</key>
<string>384</string>
<key>AELRequiredCPUType</key>
<string>G4</string>
<key>AELMinimumCPUSpeedSingle</key>
<string>350</string>
<key>AELRequiredIOPCIDevice</key>
<string>PCI</string>
<key>AELMinimumOSVersion</key>
<string>10.3.2</string>
<key>AELMinimumQuickTimeVersion</key>
<string>6.5</string>