FCP runs fine on my C2D Macbook. I have 2GB memory installed, so that might make a difference.
I worked on a wedding slideshow for a friend, and the C2D MB ran FCP wonderfully, while connected to a 23 ACD at 1900x1200. The fans were on, but no problems whatsoever. I even had Photoshop CS2 and iPhoto open at the same time, moving files back and forth from FCP.
For RT Extreme preview, it can handle most of the standard transitions in FCP. But overall, it performed so much better than my old PBG4 1.5Ghz.