No, an Athlon 64 will use the full CPU power and opening two instances of Vue would more than double the render time, thats if it finished at all due to running out of memory. The use of two threads means more to P4 users as it enables the CPU to be used to the max, ie. normal CPU usage + hyperthreading (basically taking up the slack). Athlon 64s run flat out anyway so it makes far less difference.