The XtremeSystems Forums member s7e9h3n got his hands on a sample of AMD's upcoming 12-core Magny-Cours Opteron. This chip is expected to be released in Q1 2010 but s7e9h3n (don't try to type his name fast!) already had the opportunity to do some overclocking, he managed to raise the clockspeed of the chip from 1.7GHz to 3GHz.
The sample has 12 cores, 12 threads, a 1.7GHz clock speed, 6MB of L2 cache (512KB per core), 10MB of L3, and a 1.04V core voltage. s7e9h3n actually seems to be running two of these CPUs in a sample Socket G34 motherboard packed with 8GB of 1,333MHz DDR3 memory.
The system successfully ran wPrime as well as a memory bandwidth test, in which it reached almost 42GB/s. (Socket G34 systems are supposed to have four memory channels, so if my math is right, 42GB/s is pretty much the theoretical maximum with 1,333MHz DDR3).