You really should go read this article, Scott over at Tech Report has a done a great job on evaluating the performance of the new dual core Opteron processors against singel core Opterons, dual and single core Pentiums, Xeon and more. 
AMD’s dual-core Opteron processors are extremely well executed on all fronts, based on what we’ve seen. AMD’s dual-core design has a technical elegance that Intel’s can’t match, and that design brings superior performance. One Opteron 175 performs slightly better than a pair of Opteron 248s running at the same clock speed, and it does so while consuming less power than a single-core Opteron 152. All in all, very impressive.

Going to a dual-core Opteron does, however, involve some tradeoffs. Fundamentally, one is giving up single-threaded performance in order to gain multithreaded performance. Whether or not this tradeoff makes sense will depend on the kind of applications one plans to run on the system. Many of our benchmarks were multithreaded, but only made use of two threads, leaving the dual Opteron 275 system looking a little pointless. The Opteron 252 system outperformed it in many of these dual-threaded apps, like media encoding. Our other tests, however, showed the Opteron 275s to be an absolute rendering powerhouse. Which processor is the better buy will depend greatly on its intended use.