The latest generation of this lovely processor is the G4, also known as the PowerPC 7400. At the time of writing, it's running at up to 500Mhz in PowerMacs. AltiVec has been renamed Velocity Engine by Apple, presumably for marketing reasons.

Cycle for cycle, PowerPCs are faster than chips based around Intel's x86 architecture, and AltiVec gives the G4 a significant boost in certain types of tasks. At the time of writing, x86 is nullifying that advantage through sheer brute force, running at up to about 1Ghz, but 800Mhz G4s are on the horizon.

The PowerPC has other advantages, however, including a smaller form factor, and more efficient power management. The fastest x86 chips are unsuitable for use in portable devices, as they generate too much heat, while Apple is able to put its fastest G3 chips into its Powerbooks, making them the fastest laptops you can buy. As yet, the G4 doesn't run cool enough to put into a laptop, but this is likely to change in the near future.

The PowerPC line has a roadmap which stretches into the future for at least another couple of generations (see Motorola's site), while Intel are planning to pretty much totally replace x86 with something else entirely.