I have the Xeon W3520 and it is identical to the Core i7 920 except that it's memory controller can handle ECC memory. There are other W35xx Xeons that are identical to their Core i7 9xx counterparts - there is no difference in performance.
Other current Xeon models are also based on the Nehalem microarchitecture and are very similar to their Core i7 counterparts, except that they support two or more CPUs on a motherboard. These chips should perform similarly to equivalent Core i7 parts on their own, but of course will support more threads when used in a multiprocessor system.