> I wondered if our Radeon driver are hardware accelerated, too, at least for 2D.

Yes, it are ;-) Except for Radeon R600 series and newer.

> Because when I grab a window on Ambient, hold down the left mouse button and throw
> the window around for some seconds then the CPU goes up to 100% on a 1.5GHz Mac mini.

Have you switched on Enhanced Display?