OS X does a good job with multiple processors. It also gives you a slightly artificial multitasking advantage since a single 867 MHz processor can get pegged at 100% CPU from any single process, but a dual processor will always have that second processor to keep things interactive even when the first is pegged. If I had to choose between one 867 or two 500, I'd personally go for dual 500 MHz processors. One of my towers has dual 500 MHz processors and is quite responsive.