The reason you don't get the full effect of the second processor is because the processors have to share the load by communicating with each other. And they communicate with each other using the system bus. Now while OS X is quite effective at dividing up different threads to be handled by each processor (OS X only supports up to 2 processors right now, not 4), it can't do everything. So, right now I believe the max you can get out of the second processor is somewhere from 75-85%, and 100% from the other. So an application designed to run on MP systems would run about equivalent to a 1.48GHz G4 (although that's a rough estimate). Running multiple applications not designed to take advantage of dual processors would yield lower results, although still respectable.
The G5 will run much better as a MP system than the G4 will, so gains will be even more substantial. I know there were plans to make the G5 a "dual-on-die" chip, which means it could have 2 processors on a single die. Then the processors would communicate at core speed instead of the bus speed, and you would be able to get the full advantage of multiple processors (100% performance out of both). I'm not sure if that was dropped though or not. If it really is a dual-on-die, it will be incredibly impressive, otherwise we'll have to probably wait for the G6.
Within the next 2 years multi-processor systems will become increasingly more efficient and practical, and probably become the norm by 2004. The G7 is scheduled for 2005... won't that be fun
Cheers,
Dak