what is it for? Workstation or Database? this might help forum members. IMO, Opteron is pretty suitable for database, as AMD's CPU's has better architecture and results for data I/O transfer.
However, you should also consider Xeon X5670 is dual hexa-core = 12 cores + 12 virtual = 24 cores. In addition, the CoreI7 also faster than Opteron the advantage is in the triple channel memory it uses and that motherboards design for it are build for.