the official AC12 system requirements are available here. note the 4GB memory limitation and 256MB onboard VRAM recommendation. apparently AC13 will be INTEL only.
regarding upgrading your macs to cater for future software releases (incl. AC13), rob griffiths raises some good points about upgrading to INTEL based macs…
“If you’re a PowerPC user, this move to Intel-only apps doesn’t mean your machine is worthless, of course. It just means that its abilities will be limited to those programs it can run today, and probably one or two more releases of the most-popular productivity applications. I would think, for instance, that the next versions of Photoshop and Office will support both platforms. Beyond that, though, the future of application development is clearly Intel-only.”
full article available here.
update: adobe has announced that adobe after effects CS4 will not support PPCs, indicating that eventually the other CS products will also require INTEL based macs.