SPEC is always a good reference for this kind of thing. Here are their data for those two CPUs.
SPEC's result numbers are a ratio of the performance of the system to that of a Sun Ultra Enterprise 2. Roughly, the computer tested is "result" times faster than a UE2. Since all computers are referenced from that one benchmark, you can divide the results from two different computers and find their relative performances.
The CINT benchmark is integer-math based and the CFP benchmark is floating-point based. The "Rate" benchmarks test a fully loaded system and the non-"Rate", "Speed", benchmarks test a single process. That is, how fast can it do one thing versus how fast can it do a bunch of things at once.
You can find more data about SPEC's benchmarks on their web site, including information on the CPU2006 benchmark.
The Xeon is slightly faster, despite its slower clock speed. This is probably due to the Xeon's on-die memory controller, and the fact that it has HyperThreading, as shown by the fact that its "Rate" benchmarks show a greater improvement over the Core2 than the "Speed" benchmarks.