My dual 2.8GHz Xeon at work idles at 200W and peaks at 310W When fully loading both CPUs and thrashing both disks.
I don't have figures to hand, but I did measure some comparable AMD opteron systems, and they idle at significantly lower power power consumption (although that may vary by OS, depending how well it handles power management), and outperform the Xeon system.
At home, I use an old P160 as a fileserver, and that runs at between 40 and 60W depending on system load.