The main features of the PIV and the Xeon workstations is hyperthreading. It basically, in laymans's terms, does multi-threaded functions as fast as two of the same type of processors without hyperthreading, sort of. It's not exactly as fast as two processors with a multi-threaded application, but it's close. Also, the Xeon is basically just a PIV that can work as a multi processor configuration. The PIV by itself is not capable of running as a DP unit, thus all DP Intel workstations have the Xeon. I'm not sure about the Itanium configurations though. I'm pretty sure you'd be quite happy with a PIV with hyperthreading. Get what you need.
Right now, the PPC 970 (G5) does not yet have hyperthreading. IBM is planning to implement hyperthreading in their next processor design (PPC 980) by next year. I'm kind of anxious to see what the G5 can do now though, as far as rendering goes. Hopefully after they are released, we'll start getting some more accurate feedback from the actual performance of the G5.