The systems are not completely the same, but they are in the same general ballpark, so comparison is possible. As you can see, with power management enabled, under idle conditions the Opteron and the Xeon Woodcrest have similar power usage. Under load, however, the Woodcrest uses less power; this can probably be attributed to the micro/macro-op fusion mechanism used in the Core 2 architecture, which significantly increases the efficiency of each core.
The Xeon 5160 consumes 17% less power than the Opteron under load, while the Xeon 5080 consumes 30% more power. In comparing the power consumption of the two Xeon processors we see that the new Core 2 architecture of the Xeon 5160 consumes 25-35% less power than the Pentium D (Netburst) based Xeon 5080. These results seem to validate Intel's claim of 20% less power consumption.
It must be noted that in the Intel systems, every FB-DIMM consumes nearly 11 W and has its own heat sink for cooling. At over 40 W, the RAM consumes a large amount of power and thus limits the benefits of the lower power usage of the Xeon Woodcrest CPUs.