Xeons are based on the C2D but with some extra optimization for stability and 24/7 offering, along with multiple socket support. Also work with more server orientated Chipsets, that offer greater memory support, always FB-DIMM or ECC type so no cheap DDR2 and additional expansion type such as PCI-X where Server expansion cards still tend to be based on. Also usually find SCSI or SAS rather then SATA and IDE.
Comparing a Xeon with a Core isn't really a comparison as aimed at different markets. Hence why the Mac Pro uses Xeon rather then Core processors. However if you took a 2.4GHz Xeon it would giver very similar performance to a Q6600, probably slightly slower as the FB-DIMMS or ECC would impact slightly on the test, however that isn't the CPU but the memory.
If buying a server for work purposes that was critical I would always go with a Xeon/Opteron rather then a Core/Athlon system.