My iMac has a GeForce4 MX, so, going by Intell's information, not even close.
(Mine's the 1.0GHz 15" model.)
Since this thread is about all matters of speeding up Leopard/PPC, I thought I would contribute something to it (though not exactly news): don't bother watching YouTube videos via the website. Rather, use
MacTubes. It can search YouTube by itself, or take URLs from the site. It uses QuickTime to play the videos, which is far better than the dreadful Flash Player 10.1 for PPC. I tested against a random video I found on the front page; in the Flash-based web page, the CPU was pegged at 100% and still struggled on the 144p setting. (Any higher, forget it.) In MacTubes, the same video played perfectly at 360p/480p (it doesn't say which) and the CPU never went above 80%. I tried HD quality, and while it would display, I couldn't get more than 1 frame per second on a 1Ghz G4. Not really surprising, I suppose.
Note that you have to tell MacTubes to use QT, in the Player menu. It seems to use Flash by default. You need QT 7.6.4 or higher, but for some reason, Software Update in Leopard will not find/install anything higher than 7.5.5. The final PPC version, 7.7, can be found here:
http://support.apple.com/kb/dl761