In short: c't tested a 1 CPU Xserve against a comparable Dell PowerEdge 1650 (1.4 GHz P-III) running RedHat 7.2.
The XServe beat the Dell in exactly 1 test (https with keep-alive), and was between 20 to 50% slower in all others (or, as Michael would put it, the Dell was between 40% and 100% faster in all other tests).
It's still a nice server, and they even recommend it if you have an Apple-centric network, but those numbers on the Apple site sure looked different...
