Forget about mainstream, two CPU comparisons have gone up today, first article comes from ExtremeTech, take a dive into the workstation waters with its Dual CPU Shootout: Opteron versus Xeon.
In uniprocessor systems, Intel's Hyper-Threading technology often gives it an edge over an AMD64-based CPU in some benchmarks. However, once we toss in a second CPU, all bets are off. The dual Opteron system outperformed the Xeon 3.06GHz (and would likely outperform the 3.2GHz version) in benchmarks where Intel had won on single CPU performance.
If you are thinking budget then Anand's article "Clash of the 'rons" is for you, a round up of sub-$100 Celeron and Duron flavored CPUs are put to the test.