Xlr8yourmac has published some video benchmarks of the recently release iBooks. The new iBooks offer the Radeon 9550 graphics card which provides additional performance as well as support for Core Image and Core Video.
In a comparison between a 1.2GHz iBook / Radeon 9200 (old) and a 1.2GHz iBook / Radeon 9550 (new), the Radeon 9550 iBook delivered an 11% frame rate advantage in Quake 3 with sound on and a 22.9% advantage with sound off.
(Video benchmarks frequently are tested without sound to eliminate additional bottlenecks.)
Note: The actual shipping iBooks come in 1.33GHz and 1.42GHz models.