To be honest, you probably wouldn?t notice a whole lot of performance difference between the three machines when using VectorWorks for 2D stuff.
If you were to get into 3D work in the next few years, however, the G5?s advantages would become very clear cut. The geometry engine in Vectorworks 9 and 10 uses double-precision floating point maths and it just so happens that the G5 performs these kinds of calculations quicker than any current desktop processor. This exceptional ability isn?t obvious when an application performs a computation in milliseconds as is the case with VectorWorks when handling most 2D routines. However, when task durations start to span minutes and hours as is the case with certain 3D functions, the superiority of the G5 will become very obvious.
Currently, nobody really knows how great the G5?s speed advantage when running VectorWorks will be but my guestimate is that a 1.6GHz G5 would be at least twice as quick as a 1.25GHz G4. The performance deficit would be the same for the dual 1.25 GHz G4 as VectorWorks only utilises a single processor. Of course, in multitasking situations, the dual G4 would show marked speed advantages over it?s single processor G4 sibling.
The PowerMac G5?s performance advantages aren?t simply limited to the new processor either, as the new models? sport vastly superior interface, memory and video subsystems.
It is also worth noting that the new PowerMacs are much, much quieter than the current crop of machines.
The only real caveats I see in regard to the G5 are:
1. Will they ship on time or will you still be waiting for your new machine at the end of September?
2. Will Apple have ironed out all the hardware bugs in the first crop of machines?
If you still want to go down the G4 road, be aware that the dual 1.25GHz model is now discontinued and the single processor version features a year-old motherboard - ie no ATA 100, Airport Extreme, FireWire 800 or internal Bluetooth.