I was similarly skeptical about the minimum requirements for Motion until I learned more about it.
Motion would not install on my iMac G4 at work, but it would run on my iMac G5 at home. At first I thought this issue was similar to needing a fast computer to use Photoshop or edit video or mix 24 tracks of audio with unlimited effects. But as I learned to use Motion I realized that the hefty system requirements were needed because the approach to compositing was so ambitious. The application is designed to be used in real time. (I've been trying to think of an analogy, but I can't think of anything like it. And there's no way to adequately emphasize the line 'designed to be used in real time.')
Like I said, it seems to work relatively well on my iMac G5 with 2GB of RAM, but that falls quite short of Apple's suggested specs. I think Motion would really like a G5 Quad, the best graphics card money can buy, and more RAM. Put in perspective, although Motion will run on today's computers, it will probably take tomorrow's computers to make it really shine.
In the end I was glad it wouldn't install on my G4. It wouldn't do anything useful anyway. And I'm glad someone is designing apps for tomorrow. After using Motion, I reckon 2010 is gonna be pretty exciting.