>The next version of After Effects (CS4) is dropping support for PPC Macs. While this may upset some people, I think it’s better to spend development effort on new features and an improved software rather than supporting legacy hardware.
A cheap PC or Mac that will run AE just as fast or faster than an old top-of-the-line PPC Mac costs under $600. If you don’t want to upgrade your hardware you’ll need to stay on After Effects CS3 (but remember it won’t be sold anymore once CS4 is shipping.)
Read more about the reasoning behind focusing on the newer platforms on Product Manager Michael Coleman’s blog.