Turning off hardware acceleration should do the trick, just as Barry suggests.

I've found the following page from Apple really useful to see if a particular graphics card provides the necessary OpenGL 1.5 compliance that SketchUp requires:

The page you requested does not exist.

Since you're running OS X 10.5, it looks like the GMA950 only supports OpenGL version 1.2. On OS X 10.6, is supports OpenGL 1.4, which is better but not quite there.