From a whole host of benchmarks that I have seen, I would say that a MacBook Pro and DP/DC Power Mac G5 run at about the same speed (e.g., a 2GHz MBP = a 2GHz DP Power Mac G5).
Although, I just read Mac Worlds most recent review of the MBP, and the 2GHz MBP was actually slower than a dual-core Power Mac G5.