I can give you an example of the difference between a MacPro single Quad 2.8 and a FW800 Dual 1.25 G4. I was using a FW800 G4 with an ATI x850 & 7000, 1.75GB of Ram and 5 HDs to follow stocks all day on three monitors. I had Safari windows on all three monitors, Mail running, and iTunes streaming internet radio for around 7 hours a day. According to Menu meters, I was using around 50% of both processors and higher when surfing the web on the main monitor.
Now I'm using a 2008 MacPro, an 8800 & 2600, 6GB of Ram, and 5 HDs to preform the exact same tasks. I have Menu meters combining the 4 cores and it shows around 2-3% processor usage. Big difference. I can fire up Fusion and run XP Pro to stream a CNBC video, at the same time, and the usage goes up to around 8-10%. And when I go to encode a video, I'd say the MacPro is around 6 times faster than the FW800 G4.
I really don't think I needed the other Processor. My machine runs cool, quiet, and fast enough, for me. But I am kinda mad at CNBC, 'cause just after I got a Mac that could run XP fast enough to watch a CNBC video, they fixed their site to where it works fine with Macs now 😡